One other advantage of a smaller crossover is that it probably won't have to be as much of a bottle show. As epic as Crisis on Earth-X was, I couldn't help noticing how much of it was structured to take place on the show's standing sets. Part 1 had the reception at CC Jitters, parts 2-3 were largely set at STAR Labs, part 3 had the Earth-X resistance headquarters look inexplicably like the Team Arrow bunker, and part 4 was mostly aboard the Waverider and it evil twin. If this crossover is largely set in Gotham City, they won't be able to rely as much on existing sets, so having to pay fewer series-regular-level actor salaries and residuals means they'll be able to invest more in new sets. (Unless, I dunno, they set much it at CatCo's Gotham office which uses the same layout as their National City office.)
Then do a crossover show involving the Legends and Black Lightning. Why would that be so impossible for the Arrowverse producers?
You know what? I'm not going to bother arguing. I do know that I won't be watching this next crossover.