He's 31 now. While in his current form he'd be an asset to any team, for the amount he would cost, and how long he'd be effective for, two, maybe three seasons, and as players get older they become more prone to injury and take longer to recover, it would still be a big gamble.
Most do, a lot don't, in common with those who don't are the ones who are in best fitness and take the best care of themselves. Giggs, Buffon, Totti and Ronaldo fits in with them. What is key with those players in particular is how professionally they took their training from a young age, always being fit. Players who mess around with fitness till late 20s before trying to take it more seriously still do worse.
I see no reason to believe he is more likely to get injured than any other player and no reason to believe he'll only be good for another 2 or 3 seasons. Could he decline, sure, so sell him. Unlike Rooney who hasn't really been considered a star outside of England with very little international appeal, Ronaldo could play 5 years for Utd, get sold for 30mil to a MLS or Chinese team and still be a monumentally profitable star in terms of sponsorship. Wherever he goes he's mobbed, his commercial value will sustain long beyond the next few years.
Likewise, a bigger transfer fee can be offset by increased sponsorship deals and even more so the likelyhood of both doing better straight away in the CL with a player such as him and doing immediately better in the league.
150mil but he's the difference between going out at r16 and going out at semi final would be 10mil right there, difference between 3rd and 1st in the league, no direct income from tv money itself, but increased bonuses from sponsors and ability to negotiate for better deals as back to being champions, maybe 20-30mil in the shorter term again.
Yeah he'd cost a lot, but the payback would be immense, there is literally no better player you can buy right now and unlike the average 31 year old, his commercial value will maintain his transfer value many years longer than the average player.
Fact is you can buy Mbappe for £120mil and he breaks both legs in his first season never becoming a top player afterwards.