What prevents most of the miners reject such a block? Nothing. You took have an hour to verify this block, only to find out that the main chain already extended by 3 blocks and this block is orphaned, what is the point?
So as a new user joining the system, I'm not allowed to mine the blocks that I want (within the block size limit)? Don't you see how idiotic this is?
If you are a new user, you won't be able to mine any block in any meaningful time, so your only choice is to connect to any of the mining pools, and this is already the case today, mining has been separated from the full nodes client since years ago. You need some extra knowledge to setup mining pools, all the mining pools today have certain degree of customization of their code and they understand the block size limit issue much better than average people on this forum. They have overwhelming support for BIP100 since that gives them the right to decide the block size limit based on their calculation
Even if you are a new mining farm with enough R&D resources, you won't be able to construct 100MB block since there is usually only 1MB transaction every 10 minutes, maybe 5MB during peak. Unless you are an attacker intentionally construct huge blocks with lots of spamming transactions, you won't be able to construct a large block. But in that case the rest of the miners are rejecting such attacking blocks