>>Rhombus and Racket will be the same language
Exactly the point, one will have to bring Rhombus to Racket's feature parity, which by the link I posted, and the Racket people themselves acknowledge will take several years to happen. That much dev resources have to be dedicated to build a new language which will have the same feature parity as Racket. This effort could be expended to taking Racket forward. This is non trivial effort.
At the same time, you are hoping to attract users from Java/Python land(Who very likely won't come, given where your language will be(absence of killer libs, frameworks, books, q&a support etc)). While turning away existing lisp users. This ultimately becomes a disaster.
One of the biggest selling points of Racket was that it was continuously being worked on, unlike CL whose spec is frozen(putting it mildly, the real word would be abandoned).
Looks like Lisp has a lot of self destructive tendencies than anything else. Competition seems to be non existent. But Lisp communities just can't agree to work on things. A super massive pivot that just won't fix any real problems to existing users, adding very few incentives to new users, plus act as a resource drain, and cause your perfectly fine existing language to stagnate.
https://news.ycombinator.com/item?id=20503742
A similar argument is made in the above thread.
Regardless, of all this serious software also commits itself to things like stability and takes backwards compatibility seriously.