What i meant is why isn't the next miner making a SegWit-Block now. He could segreagate the signatures and making space for 4 times more transactions (and earn the fees). That is the idea of SegWit and it seems compatible with the actual rules. Nobody would reject this block. Isn't this the whole point of a softfork in comparison with a hardfork?
The point is that clients that operate with the new rules won't accept old blocks. So they won't mine on a chain with an old block that is incompatible with the new rules.
While I appreciate your descriptions, I think OP is just confused with SW's acclaimed downward compatibility as a soft fork, wondering why people should wait for reaching a majority of 90% or 80% when their
software is cool with older blocks mined in the old fashion way either before or after they start using it.
The trick is, after SW is being launched both old and new clients can work properly with each other, right, but there is one major and very important exception: The old clients can't validate segregated witness transactions, they assume these transactions always being valid.
Now suppose we get a brand new, fraudulent SW transaction in the mempool, that tries to steal coins from a wallet without access to its private key. This is what happens in next few hours:
The SW, upgraded nodes will reject it immediately and keep including healthy transactions in their blocks but If a majority or a significant portion of the network have not yet upgraded, they may validate the malicious transaction by including it in their blocks, and if it happens to be one of these old nodes's chance to pass the difficulty test, they will transmit it to their peers as their mined block ...
It leads to a chain split in the worst case or abandoning of the immature SW nodes enforcing them to back-off and use the old version, in the best scenario.
On the other hand if the old nodes are a very small minority, they will just find some of their mined blocks becoming orphan but do not react improperly, they just keep mining. In this way, the migration path will be as smooth as possible and with less pain and casualties, this is why we need such high levels of support.
