You argue that arrow towers at lvl X are cheaper than firebolt towers at lvl X and thus should be weaker? I totally agree. I did NOT say that the stats for existing arrow tower lvls should be buffed. All I suggested was to add another three upgrade lvls for the arrow tower - costing more and more, of course. And if you max out an arrow tower (currently lvl 12) you also have to spend many millions, with more upgrade levels being even more expensive. Higher lvls for arrow tower get more expensive, so what’s the problem with the increased maximum strength then?
If you’d have to pay a similar amount of gold for a lvl 15 arrow tower as you have to pay for a lvl 9 firebolt tower, then a similar strength of those two would be perfectly fine in terms of investment-reward.
And you still could choose firebolt towers over arrow towers and stick with them (without having any disadvantages), but you would also have an alternative then and be able to vary your strategies a bit more.
Some more possible diversity in tower types in high lvl gameplay really wouldn’t hurt anyone.