[SJCrew]

I think Tornadus-T is a bigger disincentive for Amoonguss usage than Genesect. Genesect doesn't get free turns on Amoonguss, since Amoonguss can tank any one of its moves while it is healthy and hit the switch-in with a status attack or wear them down with weak Giga Drain/Hidden Power damage before switching out.

Amoonguss has a lot of other problems too: it gives free turns to Ferrothorn under Rain to set up Spikes and free turns to Heatran to annoy things with Lava Plume burns and Toxic + Protect. Heatran is immensely difficult to switch into these days, so you'll want to avoid anything that gives it free turns without any sort of fight whatsoever.

Another thing to note about is that while Amoonguss is bulky, it is simply not a good general purpose wall. It cannot switch into Dragons at all and some of them can even use it for set up bait. Even using Amoonguss now, I find myself having to slap on Mamoswine to check Heatran/Dragons without losing offensive momentum (keeping in mind it cannot switch in either). 114/70/80 defenses are not good enough to take powerful neutral attacks in OU, even with investment.

Amoonguss usage will most likely stay where it is because even with its niche qualities, it is still a very limited Pokemon that does not fit on just any team. It can only check a handful of things and will often open you up to several more.