The issue is that after the emergence of L2s nobody wants to pay L1 fees while they can get the same equivalent service on L2s so Ethereum has returned to inflation in spite of the sweeping changes.
You don't get the same with an L2s, your whole argument is based on a misunderstanding. It is as if you asked me if I would use Bitcoin because I can get the equivalent or better service of using Visa.
See the thing is simple economics teach us that if a service is equivalent people can have a very high tendency to use the cheapest alternative that is essentially the same.
If the goal is to get BTC from point A to B most people won't care about decentralisation.
Yes, if visa comes up with a fully compromised way to transfer BTC that is free, many people will use it.
This is evident by how many transactions happen within any exchange. We see numbers that are up to the 10s of millions for any singular exchange. Bitcoin on chain transaction capacity can't even accommodate the number of transactions an exchange does in a day off-chain over the span of whole months.
No, because those things are not substitutes. They are complements. You are confusing the basics, and I made an extreme example to try to highlight where you error is and yet you still persisted with it.
But that's besides the point here.
The issue here is a much more fundamental one.
L2s and sidechains create a very odd situation especially if CORE decides to intergrade such functionality.
It will be economically pointless to transact on-chain while at the same time the security of the chain heavily depends on there being on-chain transactions so miners can have incentives to continue.
"Core" does not need to "integrate" anything. We already have LN and sidechains, and we will get many more. It is not "economically" pointless to transact on-chain. Again, you are making false assumptions and rolling far with them which leads to errors in conclusion.
The security assumptions and guarantees of Layer 2 are completely different, and the overall security of it is nothing compared to Layer 1 on Bitcoin. You can argue from a theoretically narrow perspective all you want, but as long as we can easily draw on counter examples of users that disprove the theory it is a moot point. I've already told you that it is never going to happen.
Read the bold point. It has nothing to do with the usability or UX of layer2s, and nothing to do with the economics either.
You do not get the core benefits of Bitcoin only because you are on a layer 2, they do not get passed down this way. Things don't work that way. Luckily nobody listened to people like you when the internet was built, otherwise they would believe that somehow the layer 5 of the internet would have destroyed layer 1.

So what are we going to rely on LN, L2s and sidechains to be hard to use forever?
Based on this do we accept that if off-chain transaction solutions become easier it's a threat or we'll just apply wishful thinking that we'll never have to face that issue because adoption will never pick up off or on chain regardless of off-chain solutions
Again, stop making a conclusion based on false premises and then try to stir panic with the assumption that your conclusion is true. There is no issue at all, and you don't have a "solution" for your "issue" either. If others don't believe that your "issue" is a thing, what is the point of talking about it? This is not any different than quantum thread, a block reward thread, or many other similar threads. The economics are only one side of the story, and when it comes to Bitcoin they are a side-effect and not a primary feature or driver of usage. A L2 can not take away utility from the base layer that it does not provide itself. It is not a substitute, it is a complement. Learn the difference.