I do think releasing Megas like Medicham will affect Metagross as, while not directly competing for his role, other viable candidates for the Mega Slot in the metagame do impact his opportunity cost to some degree.
That said, Mega Medicham definitely has a lot going for it against SuMo OU right now. Against the defensive builds it excels fighting, it continues to be absolute murder on typical cores, doubly so since Toxapex is the go-to Bulky Water and gets murdered by Zen Headbutt, which gives Medicham a bit more flexibility with coverage (if it wasn't Double Priority, I noticed it tended to be Thunder Punch). Ice Punch gives it a way to break Lando-T and Zygarde, two fairly prominent threats in the metagame right now, and it doesn't need coverage to crack Celesteela in half, which is a boon given its status as a Blanket Check. Also takes a decent chunk out of things like AV Tangrowth, and its STABs can even chunk decently bulky resist, HJK dealing around 40% to Defensive Tapu Fini for a benchmark. Tapu Lele can make it a very good partner for Psychic Spam, protecting it from priority and letting it spam Zen Headbutts only slightly weaker than HJK and with much less risk, a big deal since there are 3 notable Ghost types and Celesteela likes to run Protect. The overall slower nature of the game is nice since it makes it safer to run Adamant.
252+ Atk Pure Power Medicham-Mega High Jump Kick vs. 252 HP / 4 Def Celesteela: 376-444 (94.4 - 111.5%) -- guaranteed OHKO after Stealth Rock
252+ Atk Pure Power Medicham-Mega High Jump Kick vs. 252 HP / 4 Def Tangrowth: 319-376 (78.9 - 93%) -- 37.5% chance to OHKO after Stealth Rock
On the other hand, both Greninjas are running around to provide Fast checks, it still hates Mega Sableye on Stall teams, Dugtrio gives it Stall headaches, and Gengar being so good means that even offensive teams can potentially have a switch into HJK. Medicham remains a high-risk/high-reward Pokemon overall, but I think that the rewards it can reap and the new options to mitigate those risks mean it'll do fine for itself. Its absurd power punishes teams that depend on typical Blanket checks as their switch-ins, and the mega speed buff does it a couple favors to reduce reliance on Fake Out to Mega Evolve (something Lopunny will also probably appreciate down the line).