Their engine is not hamstringing them. Plenty of good games have shipped with Gamebryo/Creation engine, without massive numbers of bugs.

The problem is that Bethesda doesn't give a shit about fixing anything - they ship bugs that have been in previous games, that users have outright identified and fixed for said previous games. They apply the exact same we-don't-give-a-shit attitude to their engine.

Also every engine is "20 years old", Source2 has some code from GoldSrc and Quake Engine, because if the code works perfectly then you don't just rewrite it for no reason. You rewrite parts of the engine - the parts that are holding you back in some way. And Bethesda has been modifying and extending their engine.

But, ignoring all of that, suppose the engine really was the issue: it takes 5ish years to write an engine from scratch. Starfield was in development for 8 years. Skyrim released 13 years ago. Skyrim also released 2 years ago, and a couple of times in between those periods too. Bethesda could have rewritten their entire engine from scratch if they wanted to, in that time.

The problem is that Bethesda just doesn't give a shit about quality, they chose their engine development choice by development choice. The problem is that Bethesda.