The thing is, that Trumps base comes from the GOP base.
Basically let's say the GOP/Dem split is 50/50.
Trump came in, and he riled up the 30% of that split. So it's something like 30:20/50.
But now the scales are tipping. Alot of the base Trump riled up is having second thoughts. So it's something like 20:30/50. The 20% that is hardcore for Trump, will go down with Trump. The remaining 30 want to correct their course to the public.
Basically, the GOP is split between pro Trump and anti Trump. If Trump further holes himself to being the anti-establishment candidate, then that split he's created within the GOP will start infighting worse than before. Every future election will be marred with 'Is that GOP candidate an establishment lackey?'. For that base supporting Trump, it will become tainted. Even worse than Bern Bros who COULD'VE gone the same way as Trump in their fanaticism. However most of them were reeled in, more or less. Trumps base though? He's the candidate. He can't lose. Trumps assured them. If he loses, it's because it was rigged against him and the GOP didn't give their full support.
It's great. It'll damage the GOPs base with infighting, and it's already damaged the perception fo the GOP to outsiders.