Then again, negating high level spell slots with low(er) level ones was a broken and unfun tactic, so there's that to keep in mind.
Except now there is less use for low level slots, AND if you recall, we, as a group, failed to dispel a level 8 spell with level 7 spells. It's utterly broken! Also, without the possibility to use a specific spell for a specific purpose there is no reason to prepare for anything. Just take every blast in the book (those are universal!) and kill the bad guy.
But the casters who cast those spells also had to expend higher level slots to prevent you from dispelling them.
Nope, the first caster probably wasn't there at all, and the second one still had more 8th level spells than our entire party (one, maybe two, but still).
If the spell is 1 higher than your dispel you should have about a 25% chance 15+ to dispel it assuming casters of equivalent levels. For each level the spellcaster is higher than you that chance does decrease since their spell DC increases but the opposite is true the other direction as well.
PCs encounter higher level spellcasters more often than lower levels, and that guy was higher. I like my chances.
Now for the adventure it would be nearly impossible to dispel the "warning" spell because of who the caster of that spell was and also to point out to the players that maybe casting spells in the middle of a party without permission is probably a stupid idea.
So you're familiar with the adventure. And you know that at some point the sorcerer goes invisible, right? Well, I cast See Invisibility, then lied my ass off to get out of any ramifications (which mostly worked). That was a move that paid off and didn't cost much, but I'm still salty.
For see invisibility and true seeing it makes sense again why they function the way they are. See invisibility is a very specific function it sees invisible things. True seeing would let you see vs any illusion or transmutation but they put a roll requirement on it to prevent it from just always working. So if you wanted it to basically always work you prep it at a higher spell slot.
Which I wouldn't have, because, really, how often do you meet a boss spellcaster that is lower level?
Spells are no longer learn and forget. You have to think about what you want to learn, what to prep, and what level slots you want those spells to take.
They are "learn and forget" in the sense that the moment I learn a spell involves dispelling, I will forget it exists.