This is assuming they just wait to punish you. Standing in place and using shield also counters Rock Smash, from my experience. The 72 frames of Rock Smash, with 26 of those frames being endlag, is a huge deal regardless of if you're producing a hitbox or not.
Rock Smash deals 30% damage fresh, which is over two-thirds a shield, and the rock particles are likely to poke a weakened shield. It is possible to just shield a Rock Smash and punish, but many many many times my opponents just got their shield poked if their shield wasn't at full health, and I've even gotten a few shield breaks from it. Unless you know your shield is at full health, the way to punish Rock Smash was to try actively maneuvering into a position where Rock Smash won't threaten while you can remain in a position to punish (optimally behind Zard, though Zard can read this and reverse Rock Smash), or if you read Zard going to land right next to you with Rock Smash, you grab him like you would for a counter, which either way requires a lot more proactivity on your part to deal with than just seeing if the especially noticeable Pokemon Change animation happens and punishing on reaction.
You don't have to play all three equally, just competently enough to 1) not get your butt totally kicked during that time and 2) get a throw or Smash attack so you can launch your opponent far enough away that its safe to switch. If you have solid fundamentals and understand each pokemon's kits to a basic degree, you should be able to play all of them somewhat competently. Yeah I can't pull off Mega Man's crazy footstool combos or consistently kill with DHD but if I can still get a grab with them and toss my opponent offstage, I don't see why someone couldn't do that with Squirtle or Ivysaur with practice.
You really don't seem to grasp just how hard it's going to be to get back to your optimal mon against a near-equivalent or better player from disadvantage, escaping disadvantage and then winning neutral enough with a worse character to do 2 switches without taking a lot of damage or losing your stock in the meantime is no simple task. "Just grab them and throw them offstage" is going to be a lot easier said than done while you're getting juggled or are offstage against such a player. Sometimes it will work out the way you plan, just like very occasionally when Sheiks switch to Zelda in Melee to recover they can get back to the stage safely and land a Lightning Kick without getting hit to switch back without issue, but the majority of the time it's not going to work out that way and thus it's something that should be reserved as a desperation move when all your options are exhausted and you're facing probable stock loss.