Turner's Frontier Thesis is like all attempts to provide a general synthesis to explain a large region and period of time: it may or not be valid in a general sense, but specifics - the "particular," which is often the stuff of historical analysis - can always be found to prove exceptions to the rule. And then there are those who will maintain that the exceptions are so omnipresent that the synthesis cannot be regarded as valid. (Turner is less useful, for example, when it comes to the California "backwash" that flooded the Intermountain mining West and gave birth to what was practically an urban region at birth with west-to-east migration.)

The dispute between Turnerians and those who felt the synthesis did not "make a great deal of sense" as you say, became part of a debate that lasted a century. Regardless of the merits of the either side of the debate, there was no doubt that after a century, the debate had become terribly tiresome.

Western historiography has softened since the late 1980s when I was in my Ph.D. program. At that time, it was an explosive thing even to use the term "frontier" since that would declare oneself as a Turnerian. I agree with you that Turner had merit, but we can put him in perspective and understand that he proposed a synthesis that while useful in some ways his synthesis was also flawed, as is the nature of these things. At the very least, the horse was beaten well past expiration.