I could go back a few years and ask you the same question: "Hows downgrading to secure SSLv3 insecure ?" You could say "it wasn't", and then you could suddenly be facing a bunch of CVEs and a cute acronym. What makes 1.2 so special that truly, this time, there won't ever be any problems found with it? Especially when enough potential problems were found to motivate TLS 1.3?
> Regarding sslv3, why would client ever use it, downgrade/POODLE or not ?
At the time, downgrade attacks specifically forced SSLv3. So yes, why would a client ever enable downgrade attacks - intentionally, no less - is a very, very good question. Of course, we're only talking about downgrading to 1.2 these days - but we've already had the history to show us why downgrades are a bad idea, so why repeat that history?
> So what ? 1.2 is not weak. Connection is still secure.
For now. So was SSLv3, until it wasn't. At the bare minimum, it's a larger attack surface of extremely security sensitive code - which is worrying in it's own right - and an intentionally written security vulnerability which, while "not weak" today, may become weak or even exploitable in the future.
Completely future-proofing things is a losing game, of course, but seeing as insecure downgrade moots the point of 1.3 - strengthening security - it seems reasonable to try and figure out how to do things right "now" - or failing that, "before it becomes a crisis."
> Also MITM still gets to decide whether to have tls or not. Then "Why even have tls ?". If MITM is blocking a protocol (or higher version, which is equivalent), then there is nothing to be done.
Does your browser automatically downgrade HTTPS to HTTP if the former fails? I hope not! Connections failing when MITMed by unknown third parties is a feature when you're logging in to your bank. This single feature is basically the entire point of TLS, HTTPS, and the entire CA infrastructure. That's "why even have TLS". There is something to be done: Defer connecting to your bank until you're no longer on your current, terrible MITMing wifi connection. Maybe pick one of the other 20 hotspots on your connection list. As currently implemented, this requires some minor, manual intervention, on the part of the user.
TLS 1.3 with insecure downgrade accomplishes this no more effectively than 1.2. It's a waste of bits. It's why insecure downgrade to SSLv3 is gone - it was mooting the entire point of TLS.