The problem is, at least in "Modern" Sonic games, Tails's role has downgraded so badly, it's not even funny. But, then again, most of the Sonic characters have to the point where the plot of Sonic Forces pretty much was "Sonic's captured?! Welp, we're boned!" where Tails was crapping himself at the sight of a Phantom Ruby copy of Chaos 0 when he easily defeated Chaos 4 in Sonic Adventure. The previous main story in the series made him the "damsel" in distress.
This isn't a bunch of angsty teenagers writing garbage Five Nights at Freddy's fanfiction; these are grown men who have worked with these characters for years who suddenly decided to prioritize shifting gears every time a new game comes out because they're afraid of how the internet is going to react and think "We need to push the hedgehogs more" like WWE did with Roman Reigns before his leukemia came back.
Most likely, Shadow would be the 2nd Sonic character to be selected, due to ease and also relevancy due to how pushed he still is in the games. Sonic Mania was far more popular and beloved than Sonic Forces, but Sonic Team still wants you to support Sonic Forces-like projects, where characters like Tails are jokes, and will likely not be granted an opportunity to be playable in Smash Bros..