so and yet are conjunctions which can also be adverbs.
So let's start with a simple four-word De Bruijn:
so many go back: Lots of things return
many go back so: Lots of things return like that
go back so many: Move backward this amount
back so many go: Lots of things return
So nothing spectacular yet. Yet let's keep going.
yet so many go back: But lots of things return
many go back yet so: Lots of things still return like that
go back yet so many: Move backward even this amount
back yet so many go: Lots of things return again
so many go back yet: Lots of things return again, nonetheless
Yet we can do so much better (10):
yet so many go back, yet so many go back: But lots of things return, but lots of things return. (as if these things are contradictory, but they are tautology)
many go back yet, so many go back yet so: Lots of things still return, therefore lots of things still return like that
go back yet, so many go back, yet so many: Move backward now, in order that lots of things return even though they are great in number
back, yet so many go back yet, so many go: I have returned, however, lots of things return again, therefore lots of things depart
so many go back yet, so many go back yet: Lots of things return again nonetheless, therefore so many return again nonetheless (simple tautology)
The other five rotations are identical to the five yet described.
So you might see we can add more phrases with yet and so being the conjunctions. It might continue forever so.
Of these four cases, the first and last will repeat infdeinitely; it's less obvious that you can do it with the middle three, so let's try one more repetition yet:
many go back yet, so many go back yet, so many go back yet so
Lots of things still return, therefore lots of things still return, therefore lots of things still return like that
go back yet, so many go back, yet so many go back, yet so many
Move backward now, in order that lots of things return even though lots of things return, even though they are great in number
back, yet so many go back yet, so many go back yet, so many go
I have returned, however, lots of things return again, therefore lots of things return again, therefore lots of things depart
So I think we can just keep adding more repeats to this five-word phrase and the phrase will continue to be structurally sound. Whether you derive much meaning from it is another question.
Of course, there are also simpler sentences in natural English which are indefinitely repetitive; although you might consider them trivial and not in the spirit of the question, the spoken word often has inflection, tone and nuance which the written word cannot readily communicate:
No, no, no, no, no, no... no.
-- Jim Trott