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.
Usage would be higher at much higher rates of adoption, especially as a reserve currency status, even as a backbone currency. As a reserve currency many major transactions would compete to get into a block at 1mb limit, increasing usage fees and almost completely cutting out any spam. At that state the block size limit may have to be increased still.
Again, forcing most transactions off-chain also denies the benefits of Bitcoin to most of the world's population.
Why would most of the world's population not have just as much access to off-chain solutions as they do to bitcoin directly? I realize m of n oracle systems are not completely decentralized but they should be decentralized enough and they would be completely digital and global.