Bringing this out of the comments for legibility and formatting reasons:
Firebolt also upgrades at 5th, 11th, and 16th level, so at 5th level it'd be dealing 4d10 per turn, on par with a fireball albeit for single or split target.
Sorcerers have metamagic, if they take quickened spell they can cast any levelled spell alongside that 4d10 damage in return for 2 sorcery points. Wizards can still keep themselves open for silvery barbs, shield, or counterspelling (works great for power surge), or for utility spells. Of course, that also works great with power surges, or overchanelling, but works *amazing* with bladesinger, attack and then two cantrips each turn, plus a bonus action + reaction + levelled spell-cast leftover. If you have have multiclassing going on, you can go truly OP. Should be possible to get 4d10 + 4d8 + (2 different ability modifiers) by level 7 for the cost a lvl 1 and lvl 2 spell slot per ten rounds? But multi-classing has many ways to go OP just in vanilla RAW.
You're also calculating the amount this would be above just casting a cantrip, but then comparing to casting a spell each round. 1d10 doubled at 70% for 10 rounds would be 77 damage for a level 1 spell, and your bonus actions (+ ability to cast levelled spells) would still be intact. At level 5 that's 154 expected damage. That *is* spread out across 10 rounds, (not counting the casting round's 1 cast of the cantrip), but is still ridiculous economy for a lvl 1 spell. It's actually still good economy for a level 5 spell, not much out there doing 40d10 damage for one slot. Cantrips are technically unlimited economy, but the per-action damage is what makes them non-viable. Which is what you're doubling.
So looking at the trajectory: at 1st level, that 2d10 is already more (single-target) damage than other 1st level spells, and you get to use it for 10 rounds on one spell slot. It's superior to any other combat spell, and probably has non-combat uses, being double-cast cantrips.
By 5th level, it was starting to wane in usefulness, but now it deals 4d10 and it's still competitive with 3rd level spells, which you just got. Even if it cost a level 3 spellslot, it would still be ridiculous economy & flexibility.
Level 4 spellslots, level 7 full-caster, would be the first time it's not better than other spells you could cast without restricting the milieu a fair deal, ("assume you only fight groups of people that like to gather within a fireball radius . . ."). I could see it being a level 4 spell, given that it would be competing against polymorph, (trex -> average 53 damage per turn, 33 on one target, 20 on a different target, with a +10 to hit), (take in mind that polymorphing yourself makes every hit threaten your concentration, so that 53 damage + separate health pool isn't nearly as good as the numbers look without taking conc into consideration)
A good comparison is Dragon's Breath, lvl 2, 3d6 dmg breath weapon for 1 minute, upcasts to add 1d6 per spell slot. 2d10 is better than 3d6 out of the gate, but then it jumps to 4d10 two levels later without requiring an upcast, because it's the cantrip that upgrades. So it needs to be at least lvl 3 just by single-round direct damage potential in comparison to spells you can cast alongside level 2 spells, not taking into consideration flexibility of cantrip choice, (even just within damage cantrips, you can choose between attack roll or different types of saves, single target vs. (limited) multi target). And when you do get lvl 3 spells, it does 4d10 now.
Oh, and it could also be Toll the Dead for 2d12 per round instead.
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