Above a certain level, isn't the XP/gold cost trivial?
You're like the guy saying, "$4 lattes every day is a trivial cost." But $1500 per year here and $1500 per year there and suddenly you're talking about real money.
Let's put some numbers on this. Let's very conservatively estimate that the group only encounters 4 locks per level. That 80 locks in 20 levels. If you're using
knock scrolls to open them, you've just spent 12,000 gp worth of wealth. IOW, there are some pretty useful items you've passed up in order to buy those scrolls.
If you want to talk about just
knock spells, then, sure, there's probably a point in the high teens where the costs become sufficiently trivial that it just doesn't matter.
But we're explicitly not talking just about
knock. The
knock spell is being proffered as just one example of a wide and pervasive range of spells which are supposedly knocking out broad swaths of character utility from the other classes.
Let's say there are just 10 such spells. Now we're talking about 120,000 gp. Hmm... That's starting to look like a significant chunk of change.
OTOH, if we are just talking about
knock and nothing else... Well, then, I don't really care. I don't think the rogue somehow becomes instantly unenjoyable if their theoretical monopoly on lock picking has been disrupted. (I've seen plenty of rogues who didn't put any points into Open Lock in the first place.)
I've seen lots of niche-interference problems in my years of gaming. Most of them came in purely skill-based systems when two players stepped on each other's toes. Of those that I've seen in D&D, it's never been because the spellcasters have decided to throw their limited resources after problems that have limitless solutions. I'm not saying it never happens; but I am saying that these problems are pretty much fundamentally unavoidable, and going after examples of niche-interference that result only because people aren't playing the game very well is going to be a no-win scenario.
(The easiest solution to niche-interference, BTW, is to allow one or both players to rebuild their PCs to find a define a different niche for themselves.
Quickest way to screw a rogue in pre-3E? Have a second player also playing a rogue. Instant niche interference across the board.)