I do not disagree that on chain capacity needs to be more but just from what I can gather from my own observations that everybody seems to be building on L2, as long as,,, on L1 when they finalize the channels they can be sure of security and permanence. Which is still the biggest strength of Bitcoin as opposed to Ethereum and others all popularly using L2.
L2 and L1 are fundamentally different. L2 is suitable for smaller transactions while L1 is suitable for larger transactions, which makes far more sense. L2 cannot scale without the base layer being expanded as well. We will always have on-chain transactions when opening or closing the channel, which will happen far more frequently than you think. Lack of transactions on-chain will probably not be due to the popularity of L2 for years to come. There are still various hurdles to overcome.
I do think the level of security is high enough that it does not need to be much higher, just as you say to make it too expensive to attack. But it is not just about hashpower also but the diversity of hashpower that is important.
Mining is as decentralized as it gets, anything that favours economy of scale will also result in centralization. That is a fact. The security would primarily be regarding the costs of attack rather than how decentralized it actually is. If it is not desirable to attack at all, then it simply won't happen.