Not my area of particular expertise, but the answer to this is, perhaps annoyingly: "yes and no."

To my reading the "no" camp-- that is, those who thought that a great power conflict in Europe would be a long, protracted conflict-- was relatively small but included some incredibly influential people involved in war strategy and planning on all sides including Kitchener, Haig, Falkenhayn and Joffre.

Those who thought the war would be over quickly, I think, have actually been widely misunderstood. "Over by Christmas" was not some pie-in-the-sky, chauvinistic belief in victory for one's own side; it was the necessary outcome given the strategies and assumptions employed on all sides about the consequences and nature of the coming conflict.

The German strategic necessity for a short war is perhaps the best illustration of this. The logic of the German Schlieffen plan was that the war had to be over quickly. That Germany had to knock out France and then pivot to knock out Russia because a protracted two-front conflict implied a German defeat once its much larger neighbors were able to reach full mobilization. Adding Britain to the side of the entente made that logic even clearer. A British naval blockade of of Germany would not be sustainable.

The assumptions of British war planning was a precise mirror image of that German concern. British war planners believed that a kind of "economic warfare" would devastate Germany. That lack of access not only to global shipping but to global capital out of the city of London would rapidly leave Germany impotent.

Thinking from a more global perspective there were also influential thinkers who thought that the devastation involved in a great power conflict would be so great as to not be possible to continue for more than a few months. British Admiral Beatty wrote:

There is not sufficient money in the world to provide such a gigantic struggle to be continued for any great length of time.

He thought the war would be over by winter.

Jan Bloch's The War of the Future in its Technical, Economic and Political Relations, the abridged English translation of which was titled Is War Now Impossible? argued among other things that a great power war would bring about a kind of financial and economic apocalypse, and therefore couldn't be sustained.

(Mind you, he wasn't entirely wrong on that front. Britain had to back away from aspects of economic warfare plans when it became clear that it would be economic suicide. The outbreak of war nearly destroyed global financial markets and easily ranks alongside 1929 as one the great financial crises of the 20th century.)

In that sense many British thought that the war would be short, but knew that the longer it went on the more assured of victory they would be.

Politically, as Hew Strachan writes in his chapter on "The Short War Illusion":

Both armies feared that general mobilization would give rise to strikes and demonstrations. They expected domestic disaffection to deepen rather than dissipate. After all, in the 1880s Engels had anticipated with relish the possibility of a war lasting three to four years precisely because it could create the conditions for the victory of the working class."

Logistically it was not thought that basic supplies could be long enough maintained. Strachan again:

[The German general staff's] focus was less on the raw-material needs of the war industries than on the maintenance of food supplies. The initial involvement of the general staff in the issue of economic mobilization was motivated by the need to feed the army. Of related concern were the problem of liquidity (as cash was needed to buy food, fodder, and horses), and the interruption to civilian transport.... The possibility of domestic opposition to, and disruption of, its plans for war fed on the fear of socialism.

I'm less familiar with the popular understanding of a short war at the soldier's level, but Strachan writes that it was pervasive, and actually continued throughout the war, such that the end to the war was perpetually thought to be only months away even years into the conflict and that this attitude was nearly universal. This was not the result of kind of war enthusiasm or innocence that was then supplanted by disillusionment, however, so much as popular inability to conceive of the long war.

I think you'll agree that it's easy to see how many of these arguments might have seemed compelling, so I think it's also worth examining why they were wrong. As I already mentioned, one reason was that actually going ahead with total economic war nearly proved to be economic suicide for Britain and could not actually be carried out. German access to capital was reduced, but still possible via banking houses in neutral countries, which thrived during the war. Likewise, British shipping to neutral countries which then ultimately ended up in Germany was extensive.

Virtually all of the "knockout" war plans or battles on all sides failed, including the Schlieffen plan, the British attempts to force the Straits at Constantinople, the confrontation between the British navy and the German High Seas Fleet, and so on. In the case of the Ottoman campaigns which I'm most familiar with, the British simply constantly underestimated Ottoman capabilities and overestimated their own.

Michael Neiberg also makes what I think is a great point which is that because virtually all sides viewed the conflict as defensive, there really were no clearly articulated strategic goals in the conflict. In Germany the scale of the conflict meant that in order to compensate for its losses, its war aims had ballooned to such proportions that only a total defeat of the enemy could possibly accomplish them and made compromise impossible. It also explains why virtually every socialist party in Europe actually backed the war, thus delaying or averting the socio-political socialist nightmare that conservative war planners so feared.

Source wise:

Hew Strachan's The First World War, specifically as I said the section on the short war illusion.

Nicholas Lambert's Planning Armageddon, and his lecture "The Short War Assumption".

That Michael Neiberg lecture that I linked too is excellent and that entire YouTube channel, the WWI Museum and Memorial, is great.