Considering the game is stagnating technically and creatively for ages now it's not farfetched to assume the higher-ups wanted a change in personnel.
It's highly unlikely his role would involve much about tech in the project (outside of creative efforts that would challenge their current technical infrastucture), and you're stating two highly personal opinions as facts. The game has also been doing just fine on the revenue side and there are much more valid reasons of "creative stagnation" other than a single directorial person in a team of hundreds of developers (fear of change, technical debt, data showing that it might not be a good idea, production capacity...), and more importantly other design directorial roles that would almost assuredly have more responsibilities in such an area if it was an issue for the company management. As to the real reasons of why they face an uphill battle nonetheless, below might give some perspective (I am actually a bit surprised how little they've been affected by Apex, which I think is somewhat strong sign of a resilient retention and enthusiastic userbase)
Do you think the fact that they haven't updated to a newer build of Unreal in about two years could be what's holding them back?
Quick googling showed that it might use version 4.16 which was already fairly far into it's life cycle iteration (my last project involving the engine shipped far, far earlier, I think 4.9 and that was already a fairly stable one that enabled you to ship decent products) and stable, so while surely some improvements and opportunities could be found on a newer version, it wouldn't be anything close to a singular reason.
The reality is far muddier and most likely is the result of thousands of various factors, such as them having gone to the market so early (in both good and bad, but most likely enabling their success) with a small team, and then growing extremely fast would cause all sorts of pains that are to hard fix (where as both Epic and Respawn had teams that had spent years together shipping games, improving things and becoming great teams and companies) as well as the pressure to deliver content fast potentially leading to having to take more shortcuts etc. Competing games have been able to take a lot of the learnings from PUBG and start on a much healthier base, and once you're a behemoth of a production train trying to move fast, create new as well as fix past speeding tickets (which is often quite painful, but necessary) you can easily be at an disvantage even if money isn't seemingly an issue.
The truth is not visible to the outside, and I can only guess some of major factors from experience (and well actually naturally being exposed to people and information about the company), but I can certainly say that it's not nowhere as simple as a singular thing (or even two).
