I'm not a big fan of niche-stomping, and I'm very pro-niche protection. My dungeons are full of locks, traps, and barriers that simply cannot be dealt with more conveniently than with a thief. Or without a thief require great expenditures of time and resources. Or at least require deep investigation and finding the right key or passcode or the other way around the barrier.
In other words, I give thieves things to do that can't easily be replaced with "Cast Lockmaster, and if it fails, bash the door down or shape a new tunnel in the wall."
Still, the Lockmaster and Locksmith spells in GURPS Magic are a little overpowering. Here is how I changed them without taking away their core usages, something that would be pretty unfair after allowing the spells in the first place. Most of the below aren't really rules changes - they're rulings on what specific countermeasures do to the spells. On the cumulative penalty was really a true addition.
Locksmith (Magic, p. 143)
As written, except this has no effect on default uses of the Lockpicking skill! Only affects DX-based Lockpicking attempts. Against a lock with meteoric pins or elements, only gives a +2 to skill. Against a fully meteoric lock, it has no effect.
Lockmaster (Magic, p. 144)
As written, except this has no effect on locks with any meteoric pins or elements. Repeated attempts are at a cumulative -1 to skill.
Moving bars, etc. is possible but requires the Manipulate spell. Done blindly, this can require a lot of fumbling . . . or just fail because the bar is latched or locked down. Magically or mechanically spying on the other side of the door is highly recommended. Unseen locks, of course, are a -5 plus range penalties - and touching the door isn't the same as touching the lock. Expect at least a -6 to try to spring concealed locks you know must be around somewhere . . .