I've already addressed all of the previous issues.
No, you're not reading the information we are trying to convey to you.
He is presently on his 3rd loan.
Those three loans being:
EFL League Two
Austrian Bungesliga
EFL Championship
They show excellent progression but they don't scream "he's ready for the first team spot at Real Madrid", do they?
How many additional loans do you think a player requires before they are deemed sufficiently developed to be a backup keeper for any club ? Give me a number.
I don't think it's a numerical thing. I think he aces it in the CH and earns himself a chance in the PL. If he looks solid in the PL he's ready to come back to United and challenge for the starting spot. Incidentally, if he does that by the end of next season it would mean he's raw but with proven potential and back at United as the same age as Lammens is now. That is to say, he's not losing time. 23 (he's currently 21, Lammens is 23) is still extremely young for a GK.
Also, why can't a player (following 3 pervious loans) now develop at his club ?
To split this into two points:
To state this clearly, number of loans doesn't matter to me. If he'd been recalled last Jan and gone back out on loan to a different Austrian team who offered us more money would the FOUR loans count for much? Does it matter?
You keep saying "develop at his club" but he's a 21 y/o GK who's playing regularly in the Championship and, by all accounts, performing very well. Do you think sitting on the Manchester United bench is better for "development" than playing regularly at a relatively high level? If so, no need for us to go back and forth on it, I disagree and I think you're going against conventional wisdom here. Playing regular football at a high level that is challenging is, conventionally, the best way to improve a player.
Plenty of opportunities throughout the year in cup fixtures,, which is something United have previously done during the preceding decade - main keeper plays in league games, backup gets plenty of starts in cup ties.
We are in only one cup this season and, in recent seasons, we've played Onana at all times. People talk about cup games but a full FA Cup / EFL Cup run is a maximum of like 15 games? How is that anywhere near enough to develop compared to a 40(?) game CH season (and cups, FWIW)? Unless you're proposing we play him in Europe, too, and assuming we go far in Europe?
The only time we split games anywhere near that level with a young goalkeeper was with late-era DDG and Henderson. In that case it was clear that we were looking to move on from DDG and Henderson was being bled into the no1 spot. I don't think that precedent (which, again, I only recall happening once) applies in the same way with Lammens and Vitek.