Blockchains does not require append-only. There are pruning techniques like Automatic Transaction Rebroadcasting coming out on high-throughput blockchains like Saito:
These techniques will eventually get ported over to Bitcoin or whatever survives long-enough to need it. We'll get 20, 50 and 100-year rebroadcasting chains. Fees will be higher if the data needs to be stored longer. Spectrum of uses from investment to cash with different degrees of value-persistence.
Historically, there has been a number of ideas proposed. Many different variants of subchains and sidechains, different types of payment channels, ecash systems and so on.
The idea that someone discovered these in 2017 seems oblivious to what actually happened. Support for payment channels with nLockTime was likely influential for implementing transactions with smart contracts in the first place. It was there from the first Satoshi release, and payment channels was described already in 2013. It probably wasn't first on the list of things to fix as long as transactions were heavily subsidized by inflation.
https://youtu.be/agppUdX9YvI?t=65
These techniques will eventually get ported over to Bitcoin or whatever survives long-enough to need it. We'll get 20, 50 and 100-year rebroadcasting chains. Fees will be higher if the data needs to be stored longer. Spectrum of uses from investment to cash with different degrees of value-persistence.