Except the artstyle, enemies and combat mechanics they share. Even themes are not that dissimilar. I wouldn't say it's nothing like it, they can focus on similarities in the marketing and get more people interested than ever before if they do it right. I understand that the main draw of Persona are the daily structure and social systems, but if they can't transfer some new Persona fans to SMT when they share the same universe they're bad at selling their product.
The narratives and characters are also quite different from Persona, with mainline SMT typically being more minimalist in that regard while playing up the Law vs. Chaos vs. Neutral focus and cast members generally being flat representations of those alignments. Being focused on post-apocalyptic settings over contemporary ones also affects the prevailing tone. At most, P5 is the closest that the series has been to SMT in years through things like dividing physical attacks into melee and gun attacks for everyone, bringing back negotiations, being set in Tokyo, and the boss gauntlet in the final dungeon. Those passing similarities aren't enough to get that same crowd to care about SMT too, though. Also, they only share the same universe with one SMT side game specifically, not mainline SMT.
Kaneko's demons are obviously something they have in common, but I don't see anyone mistaking Doi's work for Soejima's (let alone Kaneko's human designs). Press Turn led to 1-More, yet the differences between them led the flow of battle to differ a great deal.
The closest they got to attempting to reach Persona's demographic was SMT IV:A, which increased the emphasis on story and characterization while focusing significantly more on "the power of friendship" than what's typical for SMT. Even then, they still played significantly differently, and the end result was divisive among fans of both SMT and Persona alike. I'd say that you believe that making SMT appeal to Persona fans while still keeping it recognizably SMT is easier than it actually is, and is more than just a marketing issue.
