Well, we're a better team than United and have been for quite a while and Ronaldo, along other of players that still play for us, contributed a lot to be in that position. My take is that you could hypothetically sign Mbappe or Haaland and you would still struggle against to create chances for your strikers.
I'm also saying I would have him back in the context we're already carrying a lot more useless players in big wages in the likes of Hazard and Bale. I would swap those two for Varane and Ronaldo in an instant.
But yeah I get the main point, if we had to rely on a 36 year old, or something, Ronaldo on it's last legs to carry us through the season in all competitions then yeah, we would be in deep trouble for being in that position in the first place. Even in his last few seasons for us it was already a problem the guy wanted to play every single game and Real Madrid always made deep runs into the UCL, along with other competitions and him also playing a lot for Portugal meant he was already burn out in the last months of every season. It's a miracle he's still playing at his current age really.
So from what I remember he became more decisive for the team once he was better managed in terms of spreading out his minutes. Like allowing him to rest at the beginning of the season, not playing every single game he was fit and that allowed him to close the seasons in better shape. That's not something every team can afford, except for the big clubs (in theory) as they're supposed to have competent world class players in key positions. So I'd have him at his age to be part of the squad, would be more useful than other 'stars' we have that are only passengers and of course he would need to rotate or have more of a role coming as a sub.
But this is just hypothetical and my opinion, of course I'm not considering his wages or anything like that but we're Real Madrid and for Ronaldo I don't think he's gonna be playing for a lot longer. He's going to retire very soon, son long term I don't see having him as a problem.