it depends what exactly you mean by "on bitcoin". fwiw I wrote a huge post on this about a year ago so I won't try to summarize the whole thing (https://njump.me/naddr1qvzqqqr4gupzpqnwn7y4hqdtgxj9ygngkfy7drgze2qkpr002c4yj08wxhlut36eqqxnzde5xs6rjv3kxcer2vp3lc8ac9)

the simplest point I would make is that it strikes me as an obvious evolution of bitcoin's growing utility and adoption that fiat payments will be routed through it (probably lightning but not necessarily). whether the UX is that you have a "token" on one or either side representing a dollar is maybe interesting in terms of the evolution of the product but not really in terms of the underlying economics. what I argue in that article is it probably converges on being dollar ecash rather than "stablecoins" and then finally fiat disappears altogether.