I think it comes down to what power creep is. First, you have to look at expansion of a player's power over time. Obviously, the capabilities characters are given are stronger now than they were in the early stages of D&D. Fighters have gained (and lost) weapon mastery, large amounts of bonus feats, the ability to lead troops into battle, persuade enemies not to attack their allies, and even have an impact on the battlefield greater than "I swing".
To some, this would be considered "power creep" (I saw your post about spellcasters, Michael Something!).
But then there's power creep in a more closed environment. When 2e began, it was very similar to 1e in most respects. Proficiencies were a wash, since their existence further limited what weapons you could use, and worse, limited what you could do with an ability check. Now you needed a Non-Weapon Proficiency to say, read a note or swim in water, where you might not have before in 1e.
Thac0 vs. attack matrices wasn't really worth discussing for most groups. Level limits for Demihumans were better, but that's not a factor for most games.
The most immediate "buff" was that women and shorter races could be stronger than before.
But quickly, 2e added Kits. Initially, these were a mixed bag of benefits and downsides, but quickly, as more were made, the upsides became better than the downsides. Some non-Fighters could gain Weapon Specialization, for example! Obviously that's a big deal.
Pretty soon, Kits were vital for single-classed characters, as the benefits were too large to ignore in most cases.
Then the developers started working on some proud nails. They started by first attempting to weaken the Cleric, to better support specialty priesthoods. But quickly, they realized that people would rather play the core Cleric, so in stages, specialty priests became more and more powerful, until, by the later Forgotten Realms supplements, they were powerhouses who could easily snag special abilities from other classes without batting an eye!
And of course, the ever expanding lists of spells and magic items were always on the radar- they weren't all required to be used, but many groups did use them, and there was no particular attention paid to balancing magic items against one another. Did you randomly get a Murlynd's Spoon or a Ring of Shooting Stars? Who can say?
Later spells pushed the boundaries, and could do more than the ones in the PHB. Also, the Forgotten Realms books seemed very fond of multifunctional spells with more complex effects (there's many examples, but I'm going to start with Decastave and Thunderlance, which can do different things depending on what you are facing).
We eventually got new subclasses that could do wild and wondrous things, such as the Wild Mage, and to lesser extent, the Elementalist.
New Classes popped up here and there, like the several different Crusaders, or the setting specific Dualist Wizards or Gladiators, but this was mostly a magic user's game.
Psionics was added, and anyone could maybe luck out and gain psionic abilities that drastically increased their power...or not.
By the final iteration of 2e, what ability scores, races, and classes even meant was subject to debate!
Not to mention new races by the truckload, to the point I could play a Centaur Wizard or a Xichil Thief!
So by the end of the edition, it was obvious that many options existed that could make a PHB character very different in power than a character made with supplemental materials, and there was no guidance as to what you should or should not use- that was all the DM's job.
You can see this trend existing to this very day. The developers change their minds about things. What they were very conservative about in the early development cycle they may realize doesn't actually matter in the later development cycle.
Also, they have to look at what their fans want, which is traditionally, more, more, more! And certainly, books that grant new exciting possibilities sell better than ones that do not.
In the 3.5 era, many people griped about "errata" being bundled in with new books, trying to lower the power of Prestige Classes and Polymorph, or to offer "safer, saner" variants of early PHB classes. At a certain point, however, they decided this approach wasn't working, so they decided to go full gonzo.
Incarna. Psionics. Truenamers. Shadowcasters. Incantatrixes, Planar Shepherds, Warforged, Binders, Crusaders, Warblades, Swordsages, Factotums, Snowflake Wardancer Bards, Dragonwrought Kobolds, Dragonmarks- sure, some of these were setting-specific, and some were very strong (while others were just very weird), but the problems came when you started to mix and match these things.
Is my High Elf Samurai or Swashbuckler going to be competitive in a group with a Water Orc Barbarian (Whirling Frenzy variant) and a Githzerai Swordsage (with LA buyoff?).
Who knows!? Maybe, depending on the optimization skills of the individual characters (good or bad).
But ultimately, the DM has to step in and decide what is too powerful or what is not, and what is in line with the current vision of the game and what isn't!
This got very hard in the 3.5 era since you couldn't just say "PHB only" because the PHB had the Druid and game wrecking spells like Shapechange, Simulacrum, and Gate.
(Side Note- those spells still exist in 5e, to be a headache for DM's of high level campaigns!)
What I'm basically getting at here is, power creep and power seep are inevitable over the course of a game's advancement. With no attention paid to the impact of power fluctuations on older content, and no guidance to the DM's as to what is currently considered "healthy" for the game, any opinion on the topic is subjective.
I can say "power creep is bad" but then someone will be like "so, you think beastmaster rangers are ok?". Not every adjustment to power is bad or unnecessary- but if the developers don't come out and explain it, and just release "The Book of Overpowered Nonsense" to rave reviews and big sales, then that's probably a bad thing, and people who think it's cool will be confused, or even angered when the DM says "hell no, no way, I don't care what the current game expectations are! If you can't reach a dragon, you get out a bow, I'm not going to allow you to turn into a flaming ball of energy and leap into the air to attack it! Heck, for that matter, I'm still not all that keen on Firebolt! Throw darts if you need to do something in combat, pointy hat!".
