Just to throw out a few things to consider, even though I'm also inclined to believe the Houston Astros do not operate like a big market club. Having said that, my point was "should they start to act like a big market club since the team and the fans seem to have turned the corner into a big baseball market?". Be that as it may, here are some points to ponder:
1. The contract to offer Harper should be escalated in cost to allow for payroll flexibility (re: add one more starter if possible)
2. Correa has a back injury. Back injuries in baseball (or any other sport) are fickle and honestly quite a nuisance to deal with. If Correa's back is a problem for the future, you cannot build your franchise around him. If, however, you get two to three good to great years from him with Harper in the mix, you have a dynasty in the making. Your real bankable stars are Bregman and Altuve at this point, with Springer one notch below them.
3. Harper doesn't have to be a 10 year player for the Astros, he just needs to be a 10 year contract. See: Beltran, Carlos (or any other long-term player who eventually gets unloaded to another team willing to take him off your hands well before the end of the contract is reached).
4. You could also offer an "opt-out" clause for Harper with a pay-out, let's say at year six, if he wants to opt out, the Astros pay him X amount of dollars to let him walk (and save money in the interim).
What you do is continue to grow your own talent during this phase of the Astros club make-up to replace any of the players who are not key mainstays (see: Altuve and maybe Bregman). Buy the free agent player on occasion, but truly if you add Harper to Altuve, Bregman, Springer, Correa, Reddick and La Pina... Aye Dios Mio... you have a real shot at competing for the World Series almost four to five years running. It's kind of like the BoSox adding JD Martinez last year. They went from good to great overnight and were the cream of the crop in the MLB. Add Harper.... Houston jumps way ahead of the BoSox.
I think it's doable but since I don't run thing, can't tell you if anyone else thinks this way in the Astros business and baseball side.
P.S. You don't need a top of the line catcher in this scenario, just a steady one. Who knew the BoSox could be so strong with two journeyman catchers at the helm? I didn't. I would not have given them a grade above "C-" for their catching tandem. But they were serviceable enough to help them win. In the sceanario where you add Harper, a Max Stassi and/or Stubbs or Chrinos works well enough.