Casters also get proficiency up, that depends on total character level.
And higher spell slot levels still increase the character’s damage output and specific utility. Some spells - such as Cure Wounds or Hold Person - are pretty much designed to be upcast, and some others (e.g. Heat Metal) are unexpectedly great. Someone with Bard 3 / Sorcerer 4 might only have second level spells, but a level 4 Heat Metal vs. a heavily-armored target deals 4d8 per turn without a saving throw for up to 10 turns.
In the case of heat metal the better effect is the disadvantage it gives to the victim. 4d8 is not much. A lvl5 martial does this with a rusty one handed weapon, without a spell slot, two less levels, and I resource whatsoever.
And then most lvl4 spells are much more powerful or much more useful.
Multiclassing before lvl5 is usually a bad idea. You can twist the problem how you like, that’s how the game is designed.
Take a paladin 4/fithter 3, and any smite will do much, much more than your heat metal. A champion 3/berzerker 4 (you must be really stupid to do that) will still do much, much more than your heat metal. And he will be able to do it as many time as you will.
Read the spell description.
Sure, 4d8 in itself is not much, of course. 4d8 per round as a bonus action for 10 rounds with a single spell slot, however, is a lot, especially at a level where a paladin only has second level smite slots. Which is why upcasting heat metal can be extremely powerful. (And it gives disadvantage for those 10 rounds, and there’s no saving throw or attack roll involved. If you’re wearing metal armor, it will fuck you up. And if you happen to be resistant or immune to fire, well, that bard 3 / sorc 4 might have transmuted spell.)
I know the spell. I told you. Two attacks with a longsword from one hand will do more damage than that. I’m talking about a bland longsword and no ability whatsoever on top.
On average, an ability modifier of +4 is about a d8, and +5 is about a d10. 2 longsword attacks make 4d8. 2 2hander attacks make about 5d8. That’s without any resource invested.
You’re telling me that your awesome spell requires a lvl4 spell slot that you have 2 in the day to do less than any lvl5 martial would do without expanding any resources. And you need your enemy specifically to wear a metal armor, or your spell does about nothing.