True. If #BTC is only usable through L2s then BTC is in a very fragile state. All it takes is one cycle where BTC does not pass its all time high before the Ngu crowd to abandon BTC. At that point BTC is nothing special and all the L2s can quickly ported to any BTC like coin like #LTC or #BCH or even #Monero. Note BTCpay server And Bisq have peen ported to other coins so an L2 port is very doable. When that happens be prepared.
Good analogy about the ruler but you're mistaken. Every time you mine a coin, you inflate the supply in both Bitcoin and Monero and you will in both for the next 100 years in Bitcoin. Currently, AFAIK Bitcoin has a higher inflation than Monero until the next halving but you won't notice much of a difference until the halving after that when fees will likely replace inflation to pay the miners in Bitcoin. The inflation in Monero is small, less than Gold, and about the same as the amount of Monero that's lost due to lost wallets, etc. Gold has kept its value for thousands of years so that should not stop Monero from being a store of value. Gold only started to lose its store of value in the 1970s when there wasn't enough gold in circulation and it began to be fractionally reserved...the same as what will happen to Bitcoin when it gets scarce enough. But even with fractional reserves, Gold is still not a bad store of value so I don't think Bitcoin will be harmed too much.