Mass Effect only got bigger from ME3.
However, the concern is that things like:
-The ending controversy
-So much time away from the last game (4-5 years depending on if you count DLC)
-The new 'version' that this 'semi-reboot' will provide
...will hamper the franchise from growing further. ME1 was well known in certain gaming circles but that's it. ME2 was well known in gaming but wasn't the biggest. ME3, in my opinion, reached around the highest level of exposure in gaming, but only touched into it, though it did barely it the mainstream.
That's growth, but with that, came negativity. And with time, perhaps apathy.
But we can't know this for sure for another year from now. Sure Bioware may be relatively silent about MEA for now, but that doesn't mean that'll continue into the rest of this year, or that they won't put the hype on max in early next year. For all we know, 2017 may be absolutely filled with a super active marketing from Bioware that could successfully recover the decline in interest in recent years, and grow the franchise to new heights (whatever those heights may be).
I mean, technically, the only SP game flop(?) that I can think of, is DA2. And even that performed well enough to let DAI happen. ME3 still did well, as far as we know, and DAI seemed to do better than either of the previous games in the DA series. Will all of that, and future performance of MEA be good enough for EA to keep things going? I don't know. But that's almost another matter.
I do get the sense that Bioware intends on MEA to be a semi-new starting point for Mass Effect and a new sort of attempt to bring the series to the masses. That doesn't mean that it will necessarily disregard the trilogy (though aspects will), but Bioware may be taking the opportunity to resell the idea of Mass Effect. They don't 'need' their previous fanbase to be intact for that, as much as we may hate that. With a few years after the last game and a decade since the first game, that's *almost* like MEA could be considered a Next Generation, with new people exposed to the series, new expectations, and new forms of marketing that Bioware can utilize (see: shutting down the forums but likely not ignoring what social media may offer).
In terms of personal prediction, I really don't know. In 2017, the results of MEA could range from utter flop, to a new level of reputation. Sure the last couple of years, and the ME3 ending event, were not exactly kind to the franchise. But they were also not selling much (except merch) during this time anyway. Soon they'll have to sell things (games, content, merch, more) and we'll see the results then.
I don't think MEA will make the franchise as big as 'Star ____'. But it at best may reignite the potential for it, that ME1 considered, ME2 first attempted, but ME3 leveled off from.