From what I understand, the major argument against establishing a no-fly zone is that it requires attacks beyond Ukraine's borders. The first step in establishing a no-fly zone is to knock out AA defences on the ground. Since many of those defences are likely to be located within Russia's borders, it means bombs and missiles hitting Russian soil - something that's not politically possible yet.
But what are the arguments against a partial no-fly zone? By partial, I'm thinking something like western Ukraine delineated by Dnieper river. That way all the SEAD sorties would strike Kherson plus I'm guessing a 100km buffer on the other side of Dnieper. This area we're talking about is definitely not Russian soil by any stretch of the imagination. Not even Russia claims that Kherson is Russian soil - yet. Why not act now before it becomes de-facto Russian soil?
Doing this would accomplish the following:
- Send a clear message regarding support for Ukraine's sovereignty
- Free up Ukrainian AA and air force from duties in western Ukraine to be made available in the east
- Help unblock Odessa's port
- Air tankers for Ukrainian jets
- Better AWACS intel by getting closer to the front line
- Introduce a layer of deniability regarding what air assets are flown by Ukrainians and which assets are flown by allies. (Plausible deniability for limited strikes that are required to hit Russian soil)
- Waste Russian SAMs by forcing them to shoot long range with low hit probabilities while revealing their positions for Ukrainian attacks up close.
- Alleviate some of the worries about Belorussian re-invasion from the north
- Destroy Russian war assets including AA equipped ships (all that don't get out of the way), SAM systems and aircraft.
- Provide air cover for Ukrainian army to take back Kherson
- Setup a controlled framework for pushing east in a systematic manner where there are very fewer surprises for Russia and clear understanding for citizens of allied countries.
- When no-fly is a success, this will lay groundwork for a for setting up a limited no-go zone on the ground (ex make Kyiv a no-go zone on the ground) to protect against re-invasion.
If the counter argument is that Dnieper is still way too far east. And/or that 100 km buffer is not enough to project jets from long range SAMs, what about the same idea but even further west? Or committing only unmanned drones close to this demarcation line while keeping piloted jets out east. Even then, the exception could be low level attacks with A-10's, Apaches and such.
Back to the question. Why is it all or nothing rather than some type of a customized implementation that fits the current needs? The extreme version of this question is "why not have a no-fly zone only directly above Lviv?" While tactically insignificant, even that little gesture would be a complete win on strategic and political level. It's because even if Russia takes 90% of Ukraine, it can never hope to have whatever corner happens to be covered by a no-fly zone. Lack of hope is a powerful weapon.