Well let me speak my part on this, lol, just to clarify......
I don't think Somewhere In Time or Seventh Son are "bad" albums. In fact I really quite like SIT alot.
The so-called "uproar" with Maiden fans at the time SSOASS was released didn't really have to do with epic length (I mean could you get more "epic" than "Ancient Mariner"?? lol) I think it had to do with the fact that the music overall felt "softer" and became more progressive in that way where you listened and felt like, "Okay, where the fuck is this song going? Where's the payoff, the big hook, etc.? Jesus christ do they need MORE synths and chorused-out clean guitars???"
I saw them on that tour, in 1988. They'd play songs like "Trooper" and "Prisoner" and they had energy, hook, power..... then they'd launch into something from SSOASS and you could just feel the energy drop, and put up next to the older tunes, the new stuff felt meandering. I left that show feeling pretty bummed and it especially colored how I felt about that album as a whole.
Now, fast forward to present times.....
Being a bit older (ahem, "a bit" is stretching it, lol) and with the benefit of hindsight and having different tastes these days, I can appreciate that album much more than I did at the time. There are in fact some cool melodies and hooks on there, and the playing needless to say is excellent.
But as to whether it's "just as classic and essential as the rest of the 1980s Maiden catalog"?
Well first of all, we know this is subjective at best, so there is no definitive word on it. But I guess I can't really give a solid "yes" or "no" on the matter. I look at it more like this--
Kiss released "Music From The Elder" in 1981. Voivod released "Angel Rat" in 1992. Aerosmith released "Rock In A Hard Place" in 1982. I absolutely adore all of those albums. But what they all have in common with each other and SSOASS is that they were controversial and definitely the "departure" records for each of those bands upon their respective releases. Over time, many fans of these bands have come to accept and love these albums, and many fans today will say that "The Elder" or "Angel Rat" is just as essential and classic as the rest of the early catalog now. I am one of those people. But, the fact still remains that at the time those records were released and for a little while afterwards, they were most certainly NOT regarded in this manner by a majority of fans.
I think albums like SSOASS, and even SIT, kinda fit into their own category in this regard. They've come to earn the respect and praise of many fans since their release, and they are "essential listening" for any Maiden fan at least in terms of being more than worthy of being checked out and absorbed. Some IM fans out there think that records like "Killers" aren't necessarily "essential", I've found. That happens to be my favorite Maiden record, so I definitely disagree! But it goes to show that there isn't one definitive viewpoint on this matter.
For me personally, while I consider records like SIT, or The Elder, or Angel Rat..... to be some of the BEST music that those bands ever recorded and among my favorite records of all time, at the same time I can still kinda understand and respect the reasons why a huge portion of fans out there do not share my opinion on the matter. Because as great as the music on those records really is..... they're still too "different" and a step to the side from what the majority of fans consider to be the "truly classic" sound of those respective bands.
I think every classic, long-running band has at least one or two of those albums that can be considered "Classic and essential for being different and experimental", and I think that's more or less where SIT and SSOASS stands.
....Unlike Judas Priest "Turbo" which was just a worthless piece of shit, for the most part. LOL