If your L2 has the same security and decentralization of the L1, then it's another L1. Since it has the performance part of things, it can only have either security (but then it's centralized, so it could fully securely write bullshit, which then gets written to L1), or decentralization (and then you get to write unsecure bullshit, which has no reason to either be trusted or true, and you can write that to your L1).
It could write to L1 immediately, but your writes can still not be trusted or right.
The trilemma is not a law of mathematics with binary outcomes. It's an attempt to describe certain tensions inherent in building blockchains. A great deal of work as been done on Ethereum to balance these tensions. Rollups increase throughput because they move execution off L1, while requiring proof on L1 that L2 executed correctly.
It could write to L1 immediately, but your writes can still not be trusted or right.