While I've seen Grease used to good effect, the premise of the thread is a bit overblown, and I wouldn't normally have posted. But the idea of fighter abilities converted into spells is kinda interesting, from a comparison standpoint.
The point isn't that "Grease makes wizards better than fighters".
The point is that Grease is an example of wizard abilities scaling in ways that fighter abilities don't. And Grease is just one example.
Now, the real trick here is that if you look at the DMG monster building guidelines and actual T3/4 monsters, legendary resists, magic resistance and proficiency in saving throws gets extremely cheap at higher levels.
This means that in T3/T4 fights, creatures who ignore failed saves, creatures who have better saves, and creatures with auto-advantage on saves become increasingly common. Either that, or masses of lower CR creatures.
Masses of lower CR creatures make non-AOE disabling abilities less powerful. It does make AOE blasting (of damage or status effects) more powerful, up until they start spreading out due to either numbers or DM tactics.
Larger creatures also make AOEs less effective -- see any RTS -- because at default packing fewer creatures are in each area of effect. And higher level monsters tend to be larger, on average.
The biggest problem is it's kinda hard to do so. Action Surge is the big offender, since it doesn't require an action or bonus action to use, and there is no spell that works like that. Yah, as a spell it would be extremely strong, but if it took, say, a bonus action to cast it would suddenly be much less powerful since it would only let you cast 2 cantrips on that turn.
Action Surge is a powerful effect. It is also a 2 level dip.
A problem for most 5e characters who aren't getting 6-9th level spells is that level 6-20 is often lackluster compared to level 1-5.
Second Wind is a bit easier to compare, since it's a bonus action, self only healing effect. It would certainly be more powerful than False Life, a similar 1st level spell. In fact, in comparative amount of hit points gained, Second Wind at 20th level is somewhere in the ballpark of False Life upcasted to 5th/6th level. With better action economy and actual HP rather than bonus HP. It's still a bit wonky since spells shouldn't gain a scaling bonus like Second Wind does, but the comparison is much easier.
False Life isn't a very good spell and doesn't scale that well. Compare with Armor of Agathys, which is an ok spell, but really only if you can leverage it.
Second Wind starts out reasonably beefy; a 14 con level 1 fighter regains almost half of their HP, at a level where their HP are low. It proceeds to weaken with every level; by level 20 it is a bit more than 1/6th of their HP.
HP and temporary HP compare poorly. The typical use of False Life is to "pre buff" HP, while second wind you use to either do HD-free recovery or bring your HP up from a abyssal level. The action economy of second wind becomes increasingly crappy; unless you have nothing to do with your bonus action, it becomes a non-combat ability. False Life's action economy is much better, with an hour long duration you can burn the action out of combat to "pre-heal".
The 5e action economy means that using action resources to heal in combat is usually a bad idea, while using resources to pre-heal is much better. Even with this, false life's 5 HP/slot level scaling isn't enough to make it worth burning high level slots on.
Compare with Aid, which gives stacking max+current HP, lasts 8 hours instead of 1, and boosts 3 targets instead of just yourself, and also scales at 5 HP/slot level.
Comparing Second Wind to a
bad spell doesn't tell you much. The character with false life has better options, the fighter is stuck with Second Wind.
Finally, Indomitable would be awful as a spell. Fighters get it at 9th level, so I would assume it would be a 5th level spell that allows the recipient to reroll a single failed saving throw during the duration, then the spell would end. I don't see many spellcasters using it, even if it has no concentration. The upcasts would be granting 2 rerolls in a 7th level slot and 3 rerolls in a 9th level slot, which is incredibly weak. This is actually pretty good proof that Indomitable as written is a weak ability for the fighter to be getting at 9th level.
Yes. Making Indomitable be a legendary resistance (like the beta version was) is a no brainer.
I'd even upgrade it later; like, starting at 13th level, when you use it, until the end of your next turn you gain advantage on all saves and all attacks on you are at disadvantage.