I think because of where we are right now, paying over the odds for specific players that the manager has in mind isn't exactly a bad thing per se. United is still in the midst of a rebuild - one that has been going on for a while because of the turnover at the managerial position - which led to numerous redundancies; and crucially, we are short on the combination of quality + a bit of experience + tactical fits per the manager's detail (which is where you shouldn't err because certain managers need certain qualities in certain positions). The key is to get as many fundamental players as the manager wants to anchor the XI - irregardless of the fee, and make the team function at a somewhat appropriate level (relative to United's standard over the last couple decades) - and Morata probably falls into that category because of how critical a proper 'Mourinho 9' is to his tactics. If we buy Aubameyang, and he doesn't suit Mourinho's scheme (there's a high chance he won't because he is more of a 'forward', and in some ways quite dissimilar to the traditional #9s Mourinho has used over the years), or Immobile - and he busts like at Dortmund, you'll have to repeat the recruitment cycle again - and that will be counterproductive because you'll have more turnover and uncertainty.
Once you have those fundamental players, and he team is more developed, and you have a decent recruitment structure in place (we have been very poor in this department in recent years - hopefully, the addition of Ribalta and Guzman to Lawlor and Bout will remedy that), and start functioning like the well oiled machine we were in the not-so-distant past, you'll be at a much stronger negotiating position, and then you can afford to walk away from certain targets because the selling club wants you to pay over the odds. That's what separates current United from say Juventus or Bayern Munich - who have a fair few fundamental players to anchor the team, and have a somewhat consistent recruitment + organizational strategy in place which allows them to pick and choose in certain cases. Given that, now is not the right time to experiment because Ibrahimović was only a stop-gap, Rashford and Martial are quite inconsistent and (in realistic terms) will continue to be inconsistent over the next couple years at the very least, and Rooney is done for. Get the #9 signing wrong, and our season could be in tatters given the instability at the position, and the lack of goals from secondary scorers. If Mourinho thinks Morata will remedy that, you have to back the manager. Contrast that with say Lindelöf - where we could maybe afford to walk away given the positional strength in depth, and didn't have to pay over the odds given the nature of the club we were dealing with.