Probably another big tech would fill the gap before. Sad, but that's the most likely outcome.
I would say (and I know that is a controversial opinion this one) that the fediverse itself is a little to blame here. Not because the technical difficulties to join in that are not that bad, but because most people in the fediverse want everyone here to have a very characteristic political ideology and to be very passionate about it. Like 90% (exaggerating here)of what I read here and in mastodon are politics (more so politics from a very specific pov), even after I made a big effort to get away from political communities.
Most people have not that particular ideology or are not that passionate about it, so they take a look of what's going on here and take the sane decision of not joining in.
I actually think that's the bigger impediment for the fediverse to become mainstream.