If I was playing in your campaign I would consider a different class to play but be respectful of the structure of your world, patron and warlocks.
However, while the book doesn't explicitly say you keep everything, nothing says you lose anything either. It only speaks to gifts being bestowed, secrets learned and so forth. In fact, a final showdown between a master and student sounds like a fun campaign idea. The book actually says that the warlock learns and grows in power and not that they are simply allowed to borrow power for so long as they behave. Of course it also states that there is price to learn and grow in power. What is the price for an uncaring GOO warlock I wonder?
My sense is that once you taste power you comply with tasks and the patron's orders because you want more. In your campaign it is also perhaps due to fear of losing already learned lore or bestowed gifts. I would not tell you you are wrong. I just would not do it that way and I would not think that I am violating RAW or RAI with a different take.
Note that there are examples of patrons being unaware of or uncaring toward the conduct of their warlocks.
The PHB clearly says that one should work with their DM to determine how big a part the pact will play in the adventuring career. It sounds like you encourage players to follow this advice.
In short, I believe there are contradictory statements and vague references because one size does not fit all patrons or warlocks--and I like it that way. I like the variety. Some patrons don't care about the presence of the particular character, some are demanding.
With this variety, a DM can go in many directions and there is a lot of space for DM and player to create. As with the thrust of my main argument in the thread though, I think we often see our preferences as "the way--RAW/RAI" where the books are much less prescriptive than we selectively recall. And I think most players and DMs fall prey to this without realizing it (self perhaps included).
The reason the "However, while the book doesn't explicitly say you keep everything, nothing says you lose anything either." doesn't resonate with me is two-fold.
First, the book absolutely hammers in that the pact and agreement are the source of the power - you dont get power without the pact - period. They go hand in hand. So, the necessity of the pact is clearly and solidly established.
Similarly, the service for power is also established and even emphasized at the end as a driving force for the adventurer going out.
So, those two things even tho you present them as opposite sides of the coin - one has quite a bit of emphasis - no pact np power - pretty clear.
the other - pact over keep power - not at all really emphasized or explicitly stated.
Second, look at the cleric text in the same places... ever see the rules about losing your cleric abilities if you tick off your divinity? there are none. matter of fact there aqre a whole lot of similarities between how the cleric intro is worded and how the warlock is - including following the goals, demands, etc and how that can lead into adventuring.
So, if i were to say to my warlock players "because there is no explicit statement in your PHB intro that you can lose anything at all if you tell your patron to sod off then as Gm i must accept you cannot have those taken away from you if you do not follow your patron" then i would be forced to give the same answer to my clerics cuz they also have no such explicit reference to losing abilities.
And there is no way in my worlds that i am going to ignore tons of text linking in both cases power to the source and serving the goals of the source power etc etc etc and then tell clerics and warlocks they get to keep all their powers regardless of how they deal with their "provider."
To me i go with the weight of the evidence and not the lack of evidence.
others can do what they want.