Blue
Ravenous Bugblatter Beast of Traal
I'm mostly responding to you because you're a great lead in, I'm not picking at your response in particular.Since people do play there, why not support it?
Some people play past 20. Should the developers invest effort into fleshing out those levels in the core books? (Core is where 1-20 is currently supported, so as apples-to-apples so would 20+ in this thought experiment.)
Spells, monsters, post-epic campaign advice -- all in the same page count so displacing current material, and with the same amount of time for playtesting, so that the various tiers are playtested less than they are now.
This shows that there is an cost for supporting it: diluting the time, effort and page count for everything else.
Can I offer an alternative question, based off the Moldvay Basic I started with that only went up to 3rd level:
If I needed to pick the particular level the core books would go to, and expansions could come out for higher levels if there was enough market interest, what level range would I include in core? Again with the understanding that the more level spread, the more diluted the effort and material is due to having a set amount of developer & playtester time and page count.