Also, for example, allow them to fire back and provide endless retaliation.

I'm against titans with retaliatory fire. Endless retaliation is okay, it's a decent idea, but it's more suitable for a non-shooting upgrade, if it ever gets there.
By the way, as a completely crazy idea: give a non-shooting upgrade the "Strike from Heaven" ability. That would be a worthy alternative to shooting, but it would have its drawbacks.

then the same can be said about any unit of level 7

Well, that's true. Even skeletons can be tanks, although in this regard, they are starting to lose out to some level 6 units (specifically, ents). The rest of the level 7 tier are tanks to some extent.