For one, you're missing accounting for fleet size - Delta flies 50+ A319s while they currently only have 21 M90s (although that size will probably double with the new purchases). You can't just count F seats, you need to count F seats-per-mile or F-seats-per-flight. You're also missing that the premerger M90 was configured very uncompetitively - it had by far the lowest FC percentage in the fleet IIRC.

More importantly, though, you're missing that you can't just avoid an A319 and take one of the newly configured MD90s or CRJ700s instead. The M90 has ~35 more seats, while the CRJ700 has ~65 fewer seats - and anecdotally they seem to often fly different missions (although I admittedly have seen the M90 on my SLC-SEA example flight before).

For a true comparison you really need to look at ~75-125 seat aircraft, and in that world once the DC-9s leave the fleet the regionals will be the only ones left with a >10% FC percentage. When the A319 and DC-9 were in the fleet, we could attempt to avoid the 73G... but now we're stuck. This is a clear message from Delta that narrowbody FC is not an important investment for the company - a valid move for the beancounters but one that directly makes the competition look more attractive for Delta's domestic elites.

As for the CRJ700 fleet, while I applaud that decision I expect it was needed to stay competitive with United for BE connecting traffic. United's CR7s are already configured with both a 6 seat F cabin and a surprisingly large Economy Plus cabin.

As long as the complimentary upgrades keep flowing, I'm in it for the long haul with Delta and will continue my loyalty... Delta FC > United E+ > Delta E-. But you'd better believe that the first time I'm #1-#4 on the list and miss an upgrade on a reconfigured A319, I'm gonna spend the entire flight thinking about how I could be sitting in Economy Plus instead counting my usable SWUs...