Okay then change BIP101 so that the max block size doubles every 2.5 years. Problem solved.
Plus the change in timeframe is only the short term trend which is hardly an indication of what will happen between now and the next 20 years.
No, this does not solve anything. Basically if we had a minimum and maximum within a possible projection, while speculating, Gavin would opt for a choice near the maximum. We can not predict the future and the more we try to predict (more time, e.g. years) the more we are wrong.
Pieter Wuille actually made a better suggestion (17.7% increase per year). However, this is not that good if we start with a 1 MB limit because we would reach a maximum of 8 MB in 2030. It would be interesting if we had a initial 8 MB limit and a 17.7% yearly increase.
Regardless if Bitcoin is to remain decentralized the max block size needs to increase, and it needs to increase a lot over time. Otherwise it will become too expensive to use and people will be forced to either use the traditional banking system or centralized systems like LN and side chains. There is no way around this. There is a good chance that it might get more expensive to run a full node.
Actually I disagree, and so does Gavin (among other people). Blockchain systems do not scale properly, nor were they designed to do so. We can't just infinitely increase the block size, this will just lead to other problems. We need other solutions that are going to be built on top of the blockchain or that are going to work completely off-chain.
Demonstration of the lack of scalability with Bitcoin:
At 1 MB per block, Bitcoin has a average of ~3 transactions per seconds (see
D&T thread). Visa has an average of 2,000 tps (a maximum of 56,000). Just to reach the average that Visa handles (not the maximum, nor peak times) everyday we would need ~666.6MB blocks right now.
Just for reference, we would need ~18.6666 GB blocks to be able to process the same amount of transactions that Visa can.
Tl;dr:Bitcoin can't scale well on its own. We need to do everything, i.e. increase the block size, implement sidechains, the lightning network, payment channels. Everything.