I don't believe Mike, or Neil (or many big kit drummers for that matter) have ever played a big kit to just be like "Hey, look at how big my kit is". Many of them have evolved their drum kit over the years, to help them express and play certain things with more sounds. MP's kit started with just 6 toms, a snare and two basses, but then it GREW. It's not like he started with the Siamese monster. Neil's classic Slingerland was just two floor toms, 3 rack toms, 2 basses and a snare during Fly By Night, but later added the concert toms to allow for more creatively expressive fills. Besides, as a drummer, I get chills when I heard the full 8-tom run fills that Neil did back in the day.
I think if you have a big kit, you gotta prove you can use everything, and effectively, to the point of each piece being meaningful, and not just being a show-piece. John Bonham only had 3 toms, a snare, and a bass drums but he made it SOUND huge, and I think he's one famous drummer who effectively used what he had. Sure he could've played with as many toms as Keith Moon did but that wasn't his style. It also all comes down to style. Progressive rock/metal are often known for drummer with big kits, and as such, some drummers LEARN that style, and grow up playing big kits (like myself). But there is no reason these drummers have to be labelled any way that a stereo-type of "big kit drummers" might be. If they can play well, I think kit size shouldn't matter. If they want to prove something, they'll play a smaller kit sometimes. Really, to me, the best drummers are the ones who can sit behind any size kit and just PLAY, and play well. Adaption from kit to kit really shows what a drummer can do outside of their comfort zone. I enjoy playing a 4-piece every now and then, it's challenging and allows me to focus of the dynamics and range of what's there, rather than relying on the multiple things to express certain dynamics and range (be they drums or cymbals).
For example, I loved that Neil recorded "Malignant Narcissism" on a small kit, and that in no way hindered his playing on that song. I think it's a close-minded mentality when people thing "small kits = no creativity". Sometimes it's just the opposite. But again the kit size may depend on the genre or style you're trying to employ, and as I said, bands like DT often have the sound of big kits.
And whoever they decide to get to replace MP will probably have a relatively big kit (i.e. bigger than a 5-piece). They also have to learn how to cover the band's older songs too, which have the signature big kit sound that MP had... but that really depends on who they get. I wouldn't mind hearing old DT songs on a 6 or 7 piece kit, like NDV or Gavin Harrison use. I will miss the octabans though.
-Marc.