Q: What's wrong with "dummy" transactions to make up the difference?
A: Nothing, as long as 2 years from now you're OK with wasting 2-8GB of HDD space just because some idiot decided there should be a "minimum sized block" and you're in favor of being in direct opposition to scaling.

Q: What's wrong with waiting until the mempool supports the new block size minimum?
A: Let's suppose segwit reduces weight count by 30%. That means that it takes 30% more transactions to fill 1MB of space, compared to pre-segwit. That, also, means that when there aren't enough transactions to fill 1 block, that block will take 30% longer to fill and confirm. That further means that there would be 0 transactions left to start the next block and each successive block would take exponentially longer. This all means that, if segwit actually works, there could be times where there are 1 hour block times and 3 hours to confirm 6 blocks could become a regular thing. This is the diametric opposition to scaling.