Obviously it's a matter of perspective, but I think you'd have to go to great lengths to consider CO2 not bad in itself. Look at any random planet that has a a lot of CO2 in its atmosphere. Venus comes to mind. It rains lead over there. Only very little CO2 is needed to turn our planet into an oven. The problem is not just that the equilibrium is disturbed; the problem is that humanity totally depends on an equilibrium that's almost indistuinguishable from there being no CO2 at all; even 400ppm (0.04%!) is a minute amount. We don't need an equilibrium, we need a lot less CO2.
And while it is true that all biomass you mention is part of a balance that used to be, that balance is no longer. And we do not need a balance, we need to get rid of a humungous shitload of CO2. The simple fact that the particular CO2 that comes out of those reservoirs used to be part of a balance is completely irrelevant. Also, a LOT of the CO2 that used to be part of the biosphere anyway used to be in solid plant matter. And the thing is, it matters a lot where that C is. Carbon in plants is not a problem. CO2 in the atmosphere IS a problem. And while about 90% of the problematic CO2 levels is due to adding fossil C to the atmosphere, that's only part of the problem. The other 10% (or perhaps 20%, it's difficult to get a good number) of the problem is the conversion of biomass into CO2 by destroying what used to be nature. Chopping down forests is a good example and the the construction of reservoirs is another. Not only are we releasing lots of CO2 from the biomass that used to be somewhat locked in, we're also removing capacity for converting CO2 back into C.
So I think you're wrong. Burning wood, rotting vegetation, farting animals and bubblings reservoirs are a major part of the problem. The balance you talk about used to involve not destroying nature. Now that we have destroyed a big part of nature, we cannot just go and claim that the extra atmosphere carbon that used to be locked in biomass is not a problem because it didn't come from deep underground.
The sunrays that warm up our planet really do not care what the origin of the CO2 molecule that absorbed them was.