vintage versus modern speakers

That's all depends on your amp.
Your amp will determine what you should be considering.

are great vintage speakers (Infiniti, JBL,Sansui, Sony, etc..) from the seventies better sounding than what is available now?

Absolutely not!
And while the Japanese companies built great electronics during the 70's, their speakers were very poor when compared to some of the classic US brands like AR, EPI, KLH, etc. But even these acoustic suspension designs of yesteryear cannot hold a candle to well designed and engineered modern offerings.

The OP asked a generalized question. I will repeat here question here...

are great vintage speakers (Infiniti, JBL,Sansui, Sony, etc..) from the seventies better sounding than what is available now?

I am in the camp that says no. As a whole, modern speakers using technology derived from over the decades are much better today than what was generally available then. There are specific model exceptions, then and now, but OP asked a general question.

No different than asking if Vintage Audio sounds better than Modern Audio.

I think we sometimes let nostalgia get in the way. We can blame American Pickers for that.