For me the risk is a 2010 fiasco all over again. Recall that when Ted Kennedy died and Coakley lost the special election to Scott Brown it meant Obama went from 60 senators to 59, losing his filibuster-proof majority. If Markey dies and there's an off year special election in like 2029 with a D president and a tight 50-50 senate (all of which are very plausible at this point) there's a risk of low turnout flipping a narrow senate majority. Goodbye nominations and senate approvals. Not to make it ageism, but people in their 80s die unexpectedly all the time, and it's hubristic to think that you're somehow different. See also at least one SCOTUS justice who didn't leave when the getting was good...
So, yes, personally I'd rather have someone who has a reliable seat for the long run, while also mapping 90-95% on bog standard D issues. Moulton was the easy choice when thinking about these tail risks.