When they've done stuff like this in recent memory, they've done it across the board. Instead of lowering Galka HP and raising Taru HP, and lowering Taru MP and raising Galka MP, they just raised every races' HP and MP.
Anyway, the entire reason this is a 'problem' is because of player HP relative to BST Pet HP, right? So lower BST pet HP is the same thing as a increase to player HP in terms of relative utility. But raising player HP would most certainly be across the board, in which case, now the PLD has too much HP relative to the damage output, so the damage output has to go up, in which case now the everyone is in the same boat as they were before, except the BSTs, who now have pets which are just as killable as players, while they themselves, due to lack of DT gear for themselves, are ever worse off than previously.
Without drastic changes, there's only so many ways they can make an NM lethal or threatening. And with how mature the game is, they have to design encounters around the idea that the a group is going to consist of people with certain gear, like A/O PLDs, and Yagrush WHMs. And they don't want current-end game content to be easy to clear, so they make it difficult. And then because it's difficult, they make the rewards better than previous content, to make an incentive to do that. And then new content comes out, which has to be harder, but which needs better gear (incentives), and it just goes on and on and on. Part of the whole reason for Spark gear was fresh players getting to 99 and disliking that they had to go through the gear progression of bayld/delve/skirmish/delve gear to get up to 119 and be useful for current content. It's part of the reason RME holders are so frequently annoyed is as the power creep goes on, the RME get comparatively worse.
There are plenty of ways to address this problem that are good and don't particularly screw anyone over. However, part of the problem is inherent to the game and its mechanics, and part of the problem is the community itself. Only one of those can be addressed to any degree. I just spent an hour or so explaining to someone the relative merits of WHM/SCH to WHM/RDM. I've had to explain over and over why certain multihit WSes are better than "better" WSes. I've seen mythic monks get declines for content because they don't have Oatixur and that's "the best". You can only do so much with design changes. An ignorant playerbase, or a lazy one, cannot be rectified thus. You can lead a child to knowledge but you cannot make him think. And you can create meaningful choices in a game which promote diversity, but you cannot make a player base adapt. People will do the same old same old they always did, and the nail that sticks up gets hammered down, even if it's only sticking up in people's minds.
Restore BST to pre nerf range and make the damage vary with distance to pet.
Restore BST to pre nerf range and reduce pet HP.
Increase certain jobs' HP to compensate (and then hear the howls of people who disagree with which jobs get what).
Fundamentally change the way pet/master gear works so that it's possible for both to be functional at the same time.
Change make damage for AoE TP moves and spells fall off as distance from the target increases
Give other jobs like NIN, RUN, and DNC, more and/or more effective tanking tools
Give healer and support jobs more and/or more effective damage mitigation tools.
Design content specifically so that certain jobs make the content easier, via weakness which can be triggered by certain spells or abilities only accessible to that job as main job.
Those are all things that the devs can do. They could implement them if they wanted. Some are easier than others from a mechanical stand point. As to coding, I can't say, I'm not privy to the internal mechanics of a 13 year old MMO, but from other projects I have seen of similar age, it is likely a complete nightmare in there.
The problem is that often times these changes aren't even necessary because of an inherent limitation, they're not needed at all because the things already exist. Scherzo and Earthen Armor exist: people rarely use them. Some people don't even know they exist. An awful lot of people are completely ignorant of a lot of the game mechanics, which they cannot be blamed for in some way, since they are, by and large, completely hidden, but, at the same time, there are plenty of resources, freely available, where people can go read about Cure Power, what spells are and what they do and how they do it, MP efficiency, etc. And often there are plenty of jobs and combinations that are more than capable of completing content, but just as current follows the path of least resistance, so too does the player base. What works, works, even if something else works better, and people dislike change, and dislike having to adapt. I just spend the last couple of days working on WHM because I've given up on being able to do any relevant content on DNC.
You cannot fix problems with the player base via mechanics changes. People are going to complain about something no matter what. I'm honestly waiting for the hammer to fall on BLU, since I'm now seeing people requiring BLU for content I've not seen people want BLU for before. Several people I know have suddenly started leveling BLU and gearing and hunting spells VERY suddenly after the BST changes, and these were people who didn't even play or have interest in BST; It's just that they have seen the writing on the wall.