When I think of tulpas going mainstream (that is, even appearing on the news as a recent hype), I see a number of things happening.
For one, there would be lonely teenagers who would start making them, and then putting the blame on their tulpas for everything they might've done wrong. Even for extreme cases like when being convicted of crime, they would be able to say "it wasn't me, i was switched and my tulpa did this without me being aware of it" and this would of course cause issues if said person would be imprisoned, because moralfags from everywhere would start debating how that person is innocent and in jail.
Next, there would be such a large amount of controversy about it that it would be unbearable. If you look at #tulpa.info or the tulpa.info forums, many discussions/arguments on there are either:
A) Semantic arguments
B) Muh feelings and opinions
C) Anecdotal evidence treated as absolute proof
D) Issues that shouldn't even be issues (i.e. "halp someone impregnated my tulpa and isn't willing to pay child support")
In short, it is the relatively large number of members who do not take tulpas and tulpaforcing seriously that are ruining tulpas and their image. There are plenty of reasonable tulpamancers and tulpas out there, who will be as fair as they can be (i.e. saying "i know i can't prove my existence to you, but you can always make one yourself and have them do it for you" as opposed to "Are you implying that I'm not real!!???") when it comes to this subject, but somebody who enters the community and sees things like tulpa magick, pregnancy, marriage and other nonsensical topics that usually have nothing besides drama or speculations and empty claims behind them will
not give the reasonable tulpamancers a chance, because their image of this phenomenon has already been ruined.
So, in my opinion, even if tulpas had scientific research behind them and were a proven phenomenon with their possibilities and restrictions, I still wouldn't want them to become mainstream. The more people there are, the higher the chance is that these communities will turn to shit.
tl;dr
As of right now, I wish LESS people knew of tulpa.