Wade had his meniscus removed at age 20 while at college in Marquette, and he has had a HOF career since, even if he has had to deal with the side effects at the latter part of it. Removing a meniscus is only problematic for a long term career in which that knee will repeatedly be submitted to shock without the cushion of a meniscus. Mathieu is at the end of his career and likely not to put many more years of stress on a meniscus-less knee, and while that still will not likely exempt him from the side effects, it is not as ill-adviced as it would be for another athlete in their early 20s.
Also, whereas in decades past a full meniscectomy was the norm, nowadays they only remove the torn part and leave the remaining meniscus in place, greatly reducing short and long term side effects.
Though you're right, Mathieu should just take the safer route and not undergo meniscectomy. Is being a sub for the remaining Barca games and the Euros worth that risk?