On my first read through your comment, I thought what you're saying is uninteresting.
On my second read through your comment however, I think what you're saying is interesting.
About the fact that you wanted to narrow your scope, I think you guys did that effectively enough, but could maybe have been better, which I'll come back to.
The fire spread makes sense, however I wonder how much harder it would have been to implement in place of the current system, but I trust you had your reasons for omitting it. I do question whether it would add anything to the game. To make it balanced you'd have to drastically decrease the random placement of the fire, which would just mean you stay at the same place longer, which is less fun.
About the upgrades, you're right that it would massively increase the scope. But it would also greatly influence the identity of the game. Suddenly it's no longer an arcade game. The scoring system would break, seeing as a perfect score and a meh score are pretty close together point-wise, this could of course be fixed by making the scoring exponential. On the other hand, the way you can currently 'turn your brain off and just do the thing' lends itself well to the average 'play, buy upgrades, repeat' gameplay.
In fact, following that last conclusion, it could very well be that that's what's so jarring about the original version. You're selling it as an arcade style game, but it's missing the high skill ceiling those usually have. Instead the game is an idle-esque mindless progression loop style game, without the progression.
So if you guys were like: "We're gonna make this game with upgrades in an infinite progression loop" but then said: "You know what, let's lower our scope and not make upgrades" I think you guys missed the plank a bit. Instead you could've simplified the gameplay a bit further or done a bit less polishing, so you could add minimal upgrades, something like a speed boost, costing points, which are now permanent. Or you could've changed the genre, and add a bit more skill expression somewhere in the game.