We'd say that's mostly an issue of time. We find that one can keep up their speaking abilities in a language if they keep talking to a tutor for an hour every 1-2 weeks. So let's say you spent 30 years, learning a language every 1.5 years (let's say at an hour every day), and picking up 20 languages. If you wanted them all available at a very high level at the drop of a hat, you'd probably want to spend 10-20 hours every week doing maintenance with a tutor. So we're talking about 7 hours of weekly study (for each new language) and 10-20 hours of maintenance. So it's somewhere between a half-time and full-time job to keep them all fresh and continue learning stuff. (And you're paying tutors around $5-10/hr, so that may be an additional concern).
We think it's doable, just you've got to really love it or else you're going to burn out quickly.
You can also look at this video of Alexander Argüelles for his take on the matter, and he's wayyyy more dedicated to this stuff than any of us is. (Also we think he really prioritizes mastery, in all forms (written and spoken) of each of his languages).