If your concern (judging by the article that you’ve posted a link to) is that Monzo will be bought by a big bank so that the bank can kill it (edit - I’ve just seen that you edited your post to confirm this), then;
a) I’d hope that Monzo wouldn’t sell.
b) Assuming that they do genuinely believe that getting to a billion users is feasible, that would make their acquisition pretty costly for a bank. And they or another legacy bank would have to buy at least one of the other challenger banks too, as they have a very similar business model.
c) I’m not sure that, that would be a sustainable strategy by the banks, Monzo has got to where it is with <£15m of funding so it’s seems feasible that another entrepreneur would simply create another Monzo.
You can certainly do this though, just invest in Monzo in the next crowdfunding round (if you haven’t already)!