Tony Vargas
Legend
That's a big step up from just casting 5th level spells, not souped up at all.The spell slot thing is only partially fixed, and then only if the two classes are both spellcasters of some kind. ...
Doesn't change that a Wizard 10/Cleric 10 is still only throwing souped-up 5th level spells from both classes
And, lower-level spells retain more relevance, in 5e, since their saving throw DCs /do/ scale with proficiency, not slot level, like in 3e, and as you alluded too, you do get the higher-level slots to up-cast them.
As for fighter/caster combos, well, there's always the EK.
I could have imagined 3e handling it better with a caster-level for all classes, even those with no spells at all, who don't use it for anything, just like how pure-caster wizards still get BAB, and bad saves still slightly improve. A high level fighter who finally learns a little wizardry should find that some of his experience applies, afterall.