Samurai was not the only professional workforce during the Sengoku period (broadly speaking, from mid Muromachi to early Edo period). Another major figure people always seem to forget is the professional ashigaru - composed of men from other provinces, peasants or low-level samurai/ronins. These people do not act as proper retainers, but more so long-term mercenaries, always ready for war. I'll refer to them as "mercenary ashigarus" from now on.

People often think of ashigaru as poor peasants conscripted to give their lives away as canon folders. But this isn't entirely true. Yes, there are conscripted peasants made up of random dudes from villages grabbed to fight, but the majority of ashigaru were in fact volunteers. You might think, why the hell would they volunteer to die? And no sir, it's not patriotism. Volunteer ashigaru can get tax exemption for their villages, and whatever loot they grab from dead enemies are theirs to sell. With just a few of them, you can probably live quite a comfortable life for a few years. With enormous financial gains as the incentive, peasants flooded battlefields hoping to make a quick buck.

But these "volunteer ashigarus" were not the same as mercenary ashigarus. These mercenaries almost always compose entirely of foreigners (as in, people not from the daimyo's controlled provinces), and they are usually/sometimes led by similarly foreign people. For example, the Takeda "5 famous retainers": Yamamoto Kansuke, Yokota Takamatsu, Obata Toramori, Hara Toratane, and Tada Michiyori - they were known for their outstanding abilities in harsh circumstances, and that's only provable because they're always assigned to hard situations. These foreigners (none of them were born in Kai) always lead mercenary ashigarus to face the worst problems (for example, Yokota Takamatsu assigned to cover for Shingen's retreat against the Murakami, which led to his demise). Feudal Japan was quite regionist - in the sense that people were xenophobic to even other people from just the neighbouring provinces. Sengoku daimyos were motivated to treat their own people better than foreigners to appease them, so serving a daimyo as a foreigner was quite difficult (you're often assigned the hardest jobs and have limited career progression).

so tl;dr: no, samurai were not the only professional force.