You are just restating my point.
That's option 3. The casters using their slots to cast of the warriors to help deal with flying enemies.
Again this a high level discussion. At high levels, monsters have too much HP and deal too much damage for your warriors to sit out.
And like I stated before... the 5e designers didn't not and still don't design warriors to have a good melee and ranged attack.
One of the biggest boons of Hexadins and the saving grace of early 5e rangers was having a strong melee attack and ranged attack at higher levels.
Casting
fly every or every other fight isn't cheap.
The 5e designers can say all they want. When 5e was released. Rage didn't work with thrown weapons. Nor Reckless attack. There was no feat to get archery fighting style. Thrown weapon style didn't exist. There were no Ki-dokens, And a huge chunk of monsters at the higher levels fly, teleport, or have inhuman speeds. And a lot of HP. And all their adventures handed the magic items out.
The designers assumed that at high levels, the DM would give the warriors advanced movement, let the casters cast advanced movement, or not use a large noticeable chunk of the MM.
The designers assumed the DM hands out magic items. They just gave an out if the DM did not.
If you take magic items out of high level 5e play, you either have to change
- the monsters
- the encounter design
- or the race/class options