Funny.
But, really, cantrip and extra attack are both linear, cantrips just do dice, weapons do a die + mod, they increment at about the same points, if you're comparing weapon use by a 'pure' fighter who keeps getting extra attacks.
Attack bonus also doesn't much figure into it, it progresses at proficiency, the same for weapons or cantrips, while AC tends to also scale with CR in a similar way.
What would a quadratic fighter look like? Well, if you gave a fighter MDDs like in the early playtest, such that every attack did more damage (and some of that could be added in enabled more useful maneuvers, with the list of maneuvers as well as the number of dice increasing) as you leveled,
and gave them 5e Extra Attacks, that'd start to look a bit like it.
What would a linear wizard look like?
Level | 1st level spells | 2nd | 3rd | 4th | 5th |
1 | 2 | | | | |
2 | 2 | | | | |
3 | | 2 | | | |
4 | | 2 | | | |
5 | | | 2 | | |
6 | | | 2 | | |
7 | | | | 2 | |
8 | | | | 2 | |
9 | | | | | 2 |
etc...
Of course, you'd have a chance to trade in an old known spell for a new one every level (and via scribing, any time he encountered the text of a new spell of the level he casts or lower), so you wouldn't just be up-casting sleep forever (though you could if you wanted).
Also of course, of course, all save bonuses would scale with CR at about the proficiency rate.