Gaethje is currently a better striker than Chandler. I won't argue with that. He matches up well with Fisiev for whatever reason and as you said, that was a very short notice fight for Rafael to take. As for takedown defense, I'm not sure. Maybe Justin's is slightly better, but he sets himself up for TDs more often with leg kicks. And there's almost no way he would've survived 5 rounds (much of it on his back) like Chandler did vs. Oliveira.
Paddy fought Chandler just 6 months after the Oliveira fight and dominated him more than Charles did...and finished him. It's the only Chandler fight in the UFC where Michael didn't at least stun his opponent once. You can call it MMA Math or whatever, but it happened. I have a hard time believing a huge version of Paddy (who looks almost like a MW) won't be able to take an old Gaethje down. I mean
Luis Palomino had Gaethje in a bit of trouble on the ground in WSOF back when when Justin was in his 20s.