The main chain would be more of a backbone chain being used to settle side chains and other major transactions periodically. Bitcoin would be primarily used as a store of value and transfer of major value. This is in line with you thoughts of bitcoin being a reserve currency. We shall see how it plays out.
That can't work.
Mining has to be paid for via usage fees, not the block subsidy. The only way that works is if the cost is amortized over an very high transaction volume.
Again, forcing most transactions off-chain also denies the benefits of Bitcoin to most of the world's population.
that's not necessarily true either. with small blocks and fewer tx's, fees would increase substantially (hundreds of dollars) to hopefully compensate for diminishing block rewards. if Bitcoin is used as a world reserve currency, then tx volumes wouldn't have to be high and could be used to settle international balance of payments btwn nations.
i understand the potential role for worldwide cheap transactional volume. i'm just saying that from a teleological standpoint, Bitcoin may not evolve that way. you need to be aware of this.