i would say the block size should be doubled per 2 years, and the maximum would 256 mb, based on storage speed factor.
even though if we could afford 1 Th HDD w/ $20 in 2020, every 4096 blocks would need another HDD again. that's doesn't at all qualify the sustainable development. not only this, merchants may even increase the price of HDD, due to the excessive demand of storage, leading to unfairness distribution of storage.
i don't wanna see the electric wastage with bitcoin is higher than what government consume, we don't want a currency which wastes of energy more than what fiats does.
we want a decentralised currency, but we still need  to consider about the sustainable development in order to take the bitcoin to the top.

8 gb per block would be ridiculous for most of the newbies if they use the full node wallet.
not even me could afford such a large storage for blockchain, please consider about the newbies, majority of them would not be able to use full node wallet in the future.
i have no choice but have to use online wallet or the wallet without requiring synchronisation of blockchain to prevent the crash of my PC.

basically, bitcoin XT is centralising the full node users, forcing most of the people to use wallet which doesn't require the blockchain synchronisation, susceptible to a 51% attacks.