They won't make it legacy. I changed all mine already. I can and do find CTMs, however, those that can't find them would be as upset as they've ever been (especially the shiny ones w/ FB). I would put them at the top of your CTM priority bc it is the best rock attacker, but not bc stone edge will be gone
Are you rushing to change charge moves on Smack Down Tyranitar?
I evolved 4 Tyranitars on its community day and I’ve changed the moves on 3 of 4. The last is a 93% level 20 with SD/C. The 1% chance that Stone Edge becomes legacy would make it almost completely useless. I have 1 CTM and they’re not easy to come by.
Answers
I'd say the opposite. Shiny Tyranitar should have SD/Crunch for slightly better defensive move while showing off in gyms. All non-shiny Tyranitar with Smack Down can slowly be changed to Stone Edge.
If Stone Edge becomes a legacy move, I'd expect them to replace it with another rock charge move, so I wouldn't worry too much about how long it takes.
I’d counter by saying this game is moving GLACIALLY right now. If you get some CTMs dot your i’s and cross your t’s and change your most valuable/exclusive Pokémon to its best dps/tdo moveset.
Anything is possible with Niantic, good or bad (the current Bermuda friend zone is an example). If you are in a rural/small town/suburb I’d say coins are already available. If you are in an active raiding area just keep chipping away at tier 5s and gain some additional CTMs.
Most of all: any way I can potentially avoid a Rock Tomb Tyranitar seems worth a roll for me. Tyranitar can potentially learn Rock Tomb and I'd think it is much more likely to get that than Rock Slide. Think what you will about two bar charge moves, but I'll keep on never using Rock Tomb--offensively nor defensively.
I evolved 7 during community day.
3 got crunch, 3 got fire blast, 1 got stone Edge.
I immediately used 3 charge TMs on the fire blasts, to get another crunch and 2 stone Edge. I've left it there since.
I know I'm planning to convert some to bite/crunch as I don't have any Ttar not from community day. Was planning to convert all 4 with crunch as they seem more useful, but not entirely sure. Might end up waiting, at least on the shiny and the 98%, until I find out if any Gen 4 Pokemon get moves that make them eclipse Ttar's Rock moveset...