Telomeres do hold the important function of regulating cancer, as they prevent
most cancers from ever getting out of control. Yes, sometimes cancers mutate the ability to either generate telomerase or "fix" their telomere shortening one way or another and get entirely out of control, but for every one of those occasions, there are thousands of other cancer occurrences that eventually just simply die out or go nowhere.
To get back into the magical world of CSM, if constraints of age simply magically go poof, this mechanism is no longer at play. The many thousands of irrelevant cancers that would typically go nowhere now are a very, very serious issue. Therein lies the stupidity of trying to whisk away "ageing" just like that.
And that's just telomeres, there are like, 7 other different things involved in senescence, and i frankly have no idea the repercussions they'd have on the human body.
And that's just considering the medical repercussions of this.
If we consider things like the logistical repercussions of this, a swarm of undying locusts isn't good for anyone, including the locusts themselves. We have some ""immortal"" species in real life but none of them are inherently dangerous to the balance of the eco system or their own species (i think).