I think the mistake is in thinking about it as a zero sum game. To be fair a lot of the people who I've heard make Sunbelt Exclusive arguments come from states that currently receive 0 national attention, and so could be forgiven for being aggressive in their quest for national resources. But I think a party that allows regional variation and evenly distributes resources is the one that comes out best.
I do like the idea of investing everywhere, but the party should still identify districts/states with the most promise and try to develop them with more resources than say, Wyoming or what have you. I don't think the party does a good enough job with this. They spend way too much money on advertisements/consultants and not enough on building a permanent + sustainable ground game that can also support organic GOTV in states that may not warrant as much financial help from national orgs as other, more competitive states/districts.
I'm not a believer in that the Democratic Party's status as perpetual minority party for the past couple decades was completely avoidable - I think the Republicans were eventually going to come out on top after the 80s, but Democrats have made things worse by not maximizing their potential. In fact, maybe maximizing isn't even the best word. Sometimes Democrats seem to shoot themselves in the foot out of stupidity or greed. Look no further than the issues with the DNC, and how that was all easily avoidable.
It's so frustrating.