There are exceptions from the rule. I agree on that. I spent a lot of time playing Guild Wars solo many years ago myself. Most of the MMORPGs shine however only when they're played online. They were designed this way and that's why their solo experience is quite blank.
Any game that is "shine however only when they're played online" is just really poor design. Good design would make it work for everyone - whether online w/ other players or offline w/ AI/bots/party members.
And since you said you did "solo" this many years ago and since Nightfall does have bots/AI/NPC's/party members that you can have tag along w/ you (yes. I own all the GW pieces, but never finished it and own GW2), then all main-story and single-player content should work when offline period.
Plus, GW is actually decent w/ storytelling and stuff like that, too. Not the best, but it's at least there for solo-players.
There are say arena-based MP-style games like Quake 3 Arena, which are blast even in offline mode. More skirmish modes should allow for offline play and also offer LAN support...besides you usual tie yourself to THEIR central server stuff.
You can't tell me, there's no good games like say the MP for COD4: Modern Warfare or any modern COD just can't work offline in a skirmish mode, like say Q3A/TA does.
I mentioned Warcraft 3 several times now for a reason. When your games require multiple people and last short periods, in the range of 30 minutes, then a formal lobby isn't going to work. The time it take to look up players and get them together as a team is too difficult. The same can be said of instances like the entire MOBA genre(at least in the multiplayer aspect) or shooters like TF2, which dont function optimally when below 9 players on both sides. Meanwhile cooperative games like Artimis Bridge simulator and 4x games, where you are given games that go for hours and often multiple sessions with considerable coordination, are best served by direct Lan.
Guild Wars 1 I will 100% agree with you. Nothing in that design requires online play, but games dont work when you have a raid balanced with 30 people, and 29 of them are prescripted bots. At that point your involvement is so limited you might as well not be playing unless you design the game to give you much greater control. GW2 has a problem with non-scaling map enemies that make forming parties with friends for anything but dungeons far too easy, and dungeons requiring 5 people which makes close games with friends not very pleasant.
So yes, there are times when a game is non-functional offline without hard changing the design of large sections of the game to accommodate what is basically an entirely new gameplay system.
This is a great question. I think the most likely scenario was GOG knew some single player features were locked to online only, but went ahead with releasing it and hoping for the best.
If that's the case, their sugar-coated response thanking us for exercising the due diligence that was blatantly lacking is a wee bit rich, I think.
It also doesn't mean they understand it. Really who needs to playtest. If IO walked in and said "wlel this part of the game has online lockout, but it totally doesn't effect the game we promise," and the guy playtesting it doesn't know the game or genre(or game design) enough to recognize why removing the progression is a problem....it's easy to get past.
Also, GoG knew about the lockout, they have made it clear cosmetic and multiplayer frameworks as fine. But knowing does not equate to understanding.