You can't harm the network with sigops at 1 MB.
you can. think of the sigops as another "limit" of spam that once filled nothing else can get in
Nonsense. That is spam, and not what we were talking about. You keep creating straw man arguments.
so i can fill a blocks sigops limit easily with 7tx of (c)
Irrelevant, already known and denied by nobody. You're starting to become boring.
and although its only 7tx, and only 0.68mb of data.. no other transactions can get into the base block.. not even segwit tx's
Which you can avoid by prioritizing Segwit transactions.
Is the following mechanism correct: Legacy spam transactions would been recognized and avoided because they would "steal" them computing power for no benefit?
"Steal the computing power" is a pretty weird way to label this. I'd rather say that legacy transaction spam attacks would be recognized, and pools/miners could start prioritizing the other set of transactions.
I have no problem with that concept - only that in this case we should do that with a single hard fork (like in BIP 103) to avoid having to fork every year.
Of course. A single Bitcoin hard fork is very hard, let alone several of them.