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.
Of course I'm OK with that. Only the miner pool nodes and a few big players need to keep an integral copy, and for them, such an investment is not a problem. All normal users can use light wallets.
In fact, "when the blocks are full" and an upper bound is hindering the system, changing this former upper bound against which the demand is hitting, by a lower bound, is not going to waste much in the hypothesis of continued adoption. If 1 MB blocks are most of the time full (they are), then requiring a 1 MB LOWER bound will only waste those few differences of those few smaller blocks than 1 MB (which are almost not there, otherwise the 1 MB limit wouldn't be a problem).
So the "wasted space" is limited by changing the former upper limit which became to small and should normally naturally be crossed by changing it into a lower bound. But the nice thing is that the old and the new blocks are now fully incompatible, which is what is needed for a clean hard fork (bilateral incompatibility).
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.
As I said, no, because miners can fill them up artificially. They won't be letting their expensive mining hardware sit idle 30% of the time, waiting for people to send transactions before they can START mining. No, like now, from the moment there's a block found by a competitor on which they will build, they will make a legal block, with enough junk in it for it to be a legal >1MB block so that they are not wasting time NOT mining.
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.
No, not at all, because miners can put as much junk into a block as they wish. They can make many fake transactions, even without fee, or by paying a fee to themselves. Miners are never short of transactions if that's what they need.
They prefer of course juicy real transactions with nice big fees from other people, but if they just need to generate random transactions at no cost to be able to mine, they can always do so, unlimited.
