To add to this, we're not talking about the same situation as with MegaKhan where you basically had to donate a pokemon to the opponent first in order to switch in a check because it was both insanely strong, fast, could boost, and had a strong priority move (plus that ability which amounted to a free choice band) and then the opponent would simply switch MegaKhan out. Aegis can't really OHKO stuff that isn't weak to its moves (at least, without a WP boost), the name of its game is 2HKOs. So given its low speed lots of stuff can come in, take a SB, and then cripple it next turn before going down. At that point almost anything can come in and finish the job. So unlike MegaKanga, we're talking about situations where you can severely cripple Aegis, although it may cost you a pokemon (or it may not with some prediction), and then you can clean up later, so what you're really doing is trading 1.25 of your pokemon (because Aegis can sneak before dying), for 1 of your opponent's pokemon. Not really that bad of a deal.
As for checks to Aegislash, all of the pokemon
LT. SURGE111 mentioned actually are pretty good checks. YZard would be a counter were it not for Stealth Rocks, as it tanks a shadow ball like a champ (it's not even always a 2HKO if Aegis doesn't run Life Orb), and then threatens to OHKO through Shield Forme or can roost off damage. Excadrill may not really be that great of a check, as it goes down to shadow ball + shadow sneak, but if it comes in on Iron Head or Shadow Sneak it can severely cripple with earthquake. LO Bisharp comes in on anything except Sacred Sword and OHKOs 68.8% of the time with Knock Off. Amoonguss is actually a soft counter because it tanks anything except Iron Head from max attack Aegis and OHKOs back with Foul Play (he outslows Aegis), and recovers with Regenerator and/or Synthesis (or sleeps something). Landorus tanks a shadow ball+shadow sneak and OHKOs with EP. Another he didn't mention is specially defensive Hippo. Yes, Air Balloon sucks for it, but it can handle Aegis holding any other item, and it is good for much more than just Aegis. And of course there is Garchomp, who is in a similar boat to Excadrill except he doesn't go down to SB+SS. With LO he can also OHKO through shield forme without Adamant, though it's only 50% of the time.
These pokemon aren't exactly Numel, they're top threats in the OU meta. (well, maybe not really Amoonguss, but he's not bad either.) People will be bringing them whether Aegis is in the meta or not. With the power creep that's been happening every generation, you just can't really demand a perfect counter anymore to every possible set of a strong pokemon, or one that works without requiring prediction. Aegislash is powerful, and a top tier threat, but its checks (and soft counters) seem like they're being underestimated or thrown aside simply because they're not cold stops like, say, Skarmory is to Cokeldurr.
We're not talking about situations where you have to run "Sableye or Rocky Helmet Ferro." There are a number of really viable options here that can fit on a multitude of teams.