You'd only treat playing tennis as attack rolls on the ball if you were trying to damage it, which I presume you're not. What it would actually mean is that anyone coud eventually break a tennis ball by trying to crush it. But tennis balls do eventually break with enough use anyway, so if you do want to handle it that way then the problem just lies with the fact that we don't have a way to make attacking something have a chance of success between 0% and 5%. That seems more reasonable to me than a brontosaurus being completely unable to squash a tennis ball by stomping on it, which is what immunity would mean
I don't think it's fair to say I've moved the goalposts. My original point was that force damage is poorly defined and described in game, which I stand by whether it should be distinct from bludgeoning or not; I'm not saying all force damage should just be bludgeoning. Only the examples where it's clearly extremely similar to stuff that already does do bludgeoning. There is no actual description of the effects of force damage available, and many notable things that deal force damamge seem to be described in a way that would imply B/P/S damage. Bringing up AC is just me explaining why I think your example doesn't work. What you're describing already has a mechanic in game that is not related to damage types.
Magical B/P/S damage is pretty reliable. Magical bludgeoning is only resisted by about 50 published creatures, a good number of which are swarms, so it's still almost as reliable as it is with force damage. Not to mention that the best part about magic missile is causing multiple guaranteed concentration saves, and that still happens if the target has resistance. A bludgeoning magic missile would even get to work with the vulnerability to that damage type that most skeletons have! And if that really does make the spell too weak to be useful... okay, buff it? Have it do 1d6+1 instead of 1d4. I don't think you'd need to, but it's clearly no trouble to make it stronger. Of course, you don't need to make everything that does force damage into bludgeoning anyway. Have eldritch blast do different damage types depending on who your patron is, that'd be cool.
If you really think that losing a specific damage type that almost never interacts with resistance, immunity, or vulnerability, you could just give whichever attacks and spells you wanted a trait that says it ignores those things. That way everyone is actually clear about what it does too, rather than just expecting players to gradually learn enough of the Monster Manual and other books to realise that force does that. What we gain by dropping force damage is an easier time for DMs to properly describe injuries and more reason for everyone to actually pay attention to what damage types work against different targets (indirectly buffing sorc's metamagic feature to change damage types too, since the elemental damage types are no longer strictly outclassed by force damage). All I want for this option is a clear definition of what the damage type does that is actually supported by the flavour text and mechanics. At the moment it's weird that I'm not "supposed" to desribe steel wind strike, which literally requires the caster to wield a melee weapon, as actually cutting anything