Ronaldo, played as an out and out #9 still has 3 maybe 4 season of absolutely top class strike GPG ratios in him. You're kidding yourself if you think his output would slow down in the PL. It is, however, wholly dependent on the supply lines to him as he's a box player more than not if you want to optimise and preserve him.
His wage and the potential noses he'd put out of joint that are the biggest issues, not what you're going to get from him on the pitch. Ronaldo's 35 is the normal great players' 30; him coming down is still above the apex of all but the top few players in the world.
Hypothetically, him coming to United would wreck Rashford's trajectory and put Martial back out on the wing. It's a lot of disruption considering those are supposed to be two stars for us, now and well into the 2020's.
But there's no doubt, no doubt at all, you're getting yourself an elite performer for your money, now and for a number seasons yet.
I actually think the club would go for it because the PR aspect and marketing etc. would blow literally any other player we could land out of the water. I doubt Ronaldo would come back to England, however.