The problem with the Trump campaign is that right now he is sucking all the [media and financial] oxygen from the smaller and poorly financed conservative candidates, who might shake out early being starved of funding and media attention (which is now deliberately soft "all-Donald-all-the-time").
Bush can afford to be "sitting this out" in the middle of the pack for now and keep raking in money for himself and his and GOP-e PACs, until the field gets a lot thinner and the non-Trump polled start shifting to better-known candidates still standing (i.e., better financed) who will also start getting more "free" media exposure. So then, the polls differential between Trump and smaller field will become much smaller and the "game" for 1-2 will be on...
At that point the conservatives will have lost many of their favorite candidates. Many of them, especially those put off by Trump's crude style or business dealings or his wealth or whatever (he is getting about 25% now, which is impressive in a field of 17 but less so in a field of 3 or 4) will default to Bush - "the name we know" and "he can win" and the other candidate[s] said something nice about Hillary or said something bad about "my" candidate, etc.
Don't underestimate Bush and GOP-e — "the more the merrier" is just fine with them for now, because they are collecting money and potential allies while the "Trump show" is essentially suffocating the smaller conservative fry, because they don't get media exposure (and therefore, financial support) and "normal" people who are not paying much attention to campaigns yet don't get the chance to know them.