So, my take on it all is... Yes, of course they can save Modern Sonic. The whole reason to why Modern Sonic is seen as a "failure" is that he's never gotten the chance at center stage. Each game always has something to contrast the boost gameplay (which I actually don't mind) and it's worked to varying degrees. Let's go through the Modern Sonic line-up to prove this...
Sonic Unleashed - Werehog stages
Sonic Colours - Wisps and heavy 2D usage
Sonic Generations - Classic Sonic
Sonic Lost World - Super Mario Galaxy rip-off, no boost gameplay to be seen, wisps
Sonic Forces - Classic Sonic, Custom Character, wisps, heavy 2D usage in Modern levels (from what we've seen, anyway)
Modern Sonic's boost gameplay has never had time to itself, whether it be transformations, wisps or other characters all together. I reckon if they gave the boost gameplay a whole game to itself, it'll be sure to sell. Leave the excessive 2D sections to Sonic Mania and its eventual sequel. Give other characters their own games. Leave villains in the games they were made for. Focus on doing one thing right, instead of trying to appease everyone poorly. And for fuck sake, stop nostalgia pandering! Whoever the fuck thought it'd be a great idea to give every Sonic game Green Hill Zone needs to know that it isn't nostalgia if it's already been brought into the new era.
Another way to get Sonic back on the right track is to go down the same path Mario did, and separate the different gameplay styles into different series. Mario has a series for 3D games, a series for 2D games, a series for Mario Party, a series for Mario Kart, and a series for the RPGs. If SEGA were to do this with Sonic, not only would they be able to focus on making better games, they'd be able to focus on making the gameplay mechanics right, and they'd appease more of the fans. But, sadly, what we are getting is a game filled with every single play style imaginable, and doing each one somewhat shittily.