Hiya!
Well yes, but if you play with reduced bonus then you might decide on day 1 that the DC is lower. And 25 isn't that difficult for a high-level PC with +2 Expertise: +5 stat +6 Proficiency Bonus +2 Expertise is +13 already, and +1d4 Guidance +1d12 Inspiration makes that +22 so is an auto-success thanks to Reliable Talent. Indeed, for a mid-level PC with more modest stats, you get a base of +3 stat bonus +4 Proficiency Bonus +2 Expertise for +9 to the roll; add in 1d4 Guidance and 1d8 Inspiration and your lockpicker has +16 to the roll and Reliable talent means a minimum result of 26.
Anyway, while I dislike Expertise being based on the Proficiency Bonus, I do think it ought to be increased at later levels, much as the Fighter's Combat Styles scale with extra attacks. So, how about replacing the Feat / ASI the Rogue gets at level 10 with
Advanced Knowledge: your Expertise bonus increases to +4. You also gain proficiency in a new skill.
I can see what your point is, but I still don't see the problem. The numbers are high and can result in an "auto-success". I suppose if the typical adventuring day for the group is only needing to make that roll once or twice, then yeah, could be a problem because they would "never fail". This starts to slip back into a DM style thing; as I said, I don't "design to the PC's". This goes for everything, not just DC's. The Thief trying to steal the crown jewels would be making a LOT more checks than three or four. There would be sneaking, hiding, spotting, finding traps, disarming traps, picking locks, picking pockets...the whole 9-meters. And this is likely to take a lot of time. Guidance and Inspiration might help with the first one or two 'checks', but after that, it's all on the Thief. So a mid-level Thief might have, what, +12'ish? (+4 Dex, +4 x2 Expertise) And that's for how many skills? Two, or is it three at mid-level? With a DC of 20 to 25, that's roughly 50% plus or minus. Chances are that if the Thief has to make, say, 6 of these rolls...he's going to fail at least one or two, probably three. That makes for a pretty "bad thief", in terms of doing his job.
A lot of DM's like to take the more "cinematic" approach to DM'ing. In this, the Thief PC is expected to succeed at all the 'little things leading up to the big things'; this is where a DM will decide the DC's will be low'ish...so the Thief succeeds automatically or virtually so. But then, suddenly, as soon as the Thief gets to the Grand Chamber, all the DC's for locks, hiding, climbing, sneaking, etc, jump up by +10 to +20. Now the Thief will almost be guaranteed to fail...unless he has that Guidance, Inspiration, WhateverElse bonuses. ... ... And that's where I, with my DM'ing style, have a problem. It is a serious disconnect from "the realities of the campaign world". I mean, if the lighting and material of the room has the Sneak/Hide DC set at 30, but the other halls/rooms had it set at 20...WHY did the bad guy not just apply the same thing everywhere in his stronghold so that it was always DC 30? If the doors all had DC 18 locks...except for the one into his room, with DC 29, what is the reasoning? Any bad guy that can afford to find/get one lock at DC 29 *would* have all of his locks in his stronghold at DC 29. Cost? C'mon...unless he JUST, as in the last week, purchased the place and hadn't had time to replace them, there is no logical way. I mean, would a PC do the same thing...or would the PC factor in "Ok, we need XXXXgp's so all the locks will be DC 29"? Yeah, exactly. So why wouldn't an NPC bad guy do the same?
Anyway, this is getting a bit long-in-the-tooth so I'll sum up. By me keeping my DC's comparative to THE COMMON POPULATION of the world, where a DC 15 is "a bit tricky" for a commoner to pull off (re: will fail sometimes), and DC 25 is "quite difficult" for a commoner to pull off (re: will fail often/always), it gives my Players a solid grasp of what any perceived task's difficulty may be. If the common folk say that the Cliff's of Insanity are impossible for anyone to climb...the Players can guesstimate a DC of probably at least 22, likely 25, with the possibility of even higher. If the common folk say that the Cliff's of Insanity are impossible for all but the most skilled climber...then the Players can probably guess DC 18 to 22. If the common folk say that the Cliff's of Insanity are "really steep and dangerous, so best take your time", again, the Players can surmise somewhere in the low/mid-teens (say, 13 to 15). By approaching my world building this way, everything is consistent...at least in terms of "from a commoners point of view". In this regard, Expertise, as it is, isn't a problem because all it really does is let the Thief PC really show off his skills. I always think of Thieves as they were in BECMI for some reason...where a Thief tries to avoid melee combat at virtually all costs (crappy AC and only d4hp's, remember?). I hated that "Rogues" became "nimble damage dealers" in 3.x onward. Never sat well with me. So by sticking to the way Expertise is now, it at least tempts the Player to be, well, more "sneaky-sneak" Thief and less "stabby-stab" thief. And, IMNSHO, any rule that does that needs to be encouraged.
^_^
Paul L. Ming