If the on chain volume doesn't escalate and grow by 2x every time you will NOT have 1GB blocks!
Except that you should always expect that the blocks will be filled. In the worst, spammers will fill it with garbage. May I remind you that spammers currently pay for highest priority? I'm pretty sure there will still be around if they can spam for free.
If the chain volume doesn't grow one bit from now you will not have even 2 MB blocks, since unless I have lived in a different reality having the maximum limit at 1MB didn't make all blocks at least 1MB, it made them at most 1MB!
Doesn't work that way. If a spammer sees they can fill blocks with movies, expect movies on-chain. We've already seen this on BSV, and we all know what is its demand.
As for sustainability, the fee in reward right now is 9%, you only need 10x demand to completely override the block reward which will not be gone tomorrow. If you don't think on-chain demand will be x10 times more in 25 years then you don't have to worry about the fate of Bitcoin anymore since it's pretty obvious what that would be!
You've misunderstood the assignment. There's a
supply in demand and supply, not only demand. If you're doubling the block size every four years, transaction fees will drop
a lot. When mempool weighs less than the block size, miners are incentivized to even confirm 0.01 sat/vb. For your math to apply, demand (for on-chain volume) needs to double every four years.