Why are pro tera people so confident this will be the case? Legitimate question here.
Not them, or anyone else but myself, but I personally believe that will be the case because its about what metas do: gravitate towards what's best. It isn't the strongest of pulls, since anti-meta and general shifts and quirks do happen, but its unreasonable to think that, generally, people won't do what will make them win more often. Yes, lower ladder will be plagued by off-meta shit, but that's practically a given, tera or otherwise. Higher ladder, where more serious play occurs (as you probably know ngl just sayin for those that don't), will more likely than not use the shit that works, over the shit that doesnt.
For something less general and more topical, Tera Flying Roaring Moon is all the rage, and for good reason. It, in the Booster Energy set, allows it to not only exploit Acrobatics, but also handle the ever present Fighting type moves. Yes, its now weak to electric and rock, but those types arent, afaik, too prominent. Alternatively, as many pokemon run many sets (LANDORUS YOU BITCH) RMoon could also run Tera Fairy, which, while also handling fighting, also lets it be weak to only Poison and Steel, while having rather good coverage in Fairy Tera Blast. Yes, a few will run other tera sets, but they are far and few between. As an offensive pokemon, RMoon is mostly settled, as was the aforementioned Landorus, until someone decided to invest what, 52 EVs elsewhere? Point it, the meta is, while not super firm, already well known, and not just on RMoon. Steel, Fairy, and Water are all good defensive options to tera into, and Skeledirge, among others, uses them well.
At first we didn't know what sets would be popular in almost every generation, but with time shit settles and we get into the more familiar zone we aren't currently in.
Now, should we give it time, is a question I'll let you answer, if you wish to do so.