The few people who seem to be confusing an "engine update" with an "engine swap" in this thread seem to midjudge just what amount of effort that would entail. Sure, (insert your favourite engine here) is a nice engine, but switching engines is not a thing any sane person does for an ongoing project. You can't simply take code and copy-paste it from one engine into the other with some changes. Instead, it would be more like rebuilding the game from scratch in the new engine, to the technical specifications and limitations of that new engine. That's already basically developing a fully new game in terms of development, especially if the graphics are updated at the same time. Add to that the fact that it would have to be backwards compatible with whatever their current data structures are if you don't want the upgrade to also reset everyone's current state and progress. This means you might not even get much improvement in terms of server architecture out of it, depending on what the limiting factors are, while hugely complicating the requirements instead of playing to the strength of a new engine. It's not worth it for anyone, players or developers. Should FF XIV ever nears the end of its life cycle in a decade or two and SE makes a new MMO, they can make or use a new engine for it. Any improvements to this game will have to come from updating the current one.
But it will be as they have always done: Silently, bit by bit, in the background. Historically they have prioritised server improvements, physics and other such things over graphics when it comes to prioritising changes, because those will keep the game's look consistent and probably only be a marginal raise on the system requirements. I'd expect this trend to continue into the future. As they have stated repeatedly, the limitation on their ability to improve is know-how, not budget. I therefore also wouldn't expect the scope of the upgrades per patch to ever increase.
As for delaying content to free up time, this very thread is the perfect example as to why that would go over poorly, as no one can agree what should be cut in favour of what. To me you could remove dungeons and nothing in terms of content would be lost since those are a snorefest from the second time on when there is no new and shiny story and lore in it anymore. For others that might be the main content they run and they'd instead remove Exploration content or raids in a heartbeat cause they don't do that content. No one would ever agree and the odds of any one person ending up getting exactly the content the find expendable cut in favour of updates is next to nil.