Feels like confirmation bias to me. All of the Sonic movies have been successful, and it is common box office progression that each iterative sequel grosses more than the last, at least before franchise fatigue sets in.
Were people clamoring for Knuckles after Sonic 2 was more successful than Sonic 1? He even got his own TV show. Shadow hasn't got that. It seems like people are simply looking for why Shadow may make it in, ascribing things like a Hollywood movie Nintendo has zero involvement in or ownership of as a germane factor, when it's really not. Much like it wasn't treated to be when it would've benefitted the other Sonic characters.
In fact, it's many steps removed from what usually informs inclusion. Which isn't decided by Sega either. We don't even know how much the Mario movie is going to factor into how Smash takes shape, and that was more successful, and actually had direct involvement from Nintendo.
And here's something else that won't help. Granted it's for the first two, but it's unlikely the third really bucks the trend:
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If Sonic 4 comes out in a few years and is even more successful than Sonic 3, which is possible based on how the box office often works, but features, like... Silver... are people going to be like 'omg Silver is happening?' Not with any real momentum.
Also, Sega already knows Shadow is popular. These movies were made to reflect the biggest characters in the series.
This is just people trying to build Shadow's case by taking things that are unlikely to actually be much of a factor and donning them undue significance.