Old MMO games still in operation, all of them older than DDO, that manage to have working architecture without horrendous lag or excessive downtimes (In no particular order):
WOW
FFXI
Everquest
Everquest 2
Runescape
Dark Ages of Camelot
Ultima Online
Anarchy Online
Eve Online
MapleStory
Mabinogi
Lineage 2
Guild Wars
Perfect World
All these games have differences and unique aspects to them but all of them manage to do what, apparently, DDO cannot. Is DDO really so different and unique that it can't be compared to them fairly? I'm not a game designer or a professional software developer so I'm not going to try to deep dive into things I don't know about but nor am I going to let it shut down fair criticism either. As a consumer and customer, when I see that these games can have reasonable and smooth operations, some of them even the same parent company as DDO, I'm not interested in excuses or resigned lamentations on social media about DDO's plights.
If your equipment is crap, replace it. Just do it. Stop putting band-aids on a festering wound. We don't need countless entire days of downtime. Just bring them down, replace what needs to be replaced and don't bring it back up until it works. It's not like SSG isn't still making large profits, to say nothing for Daybreak's even deeper pockets. Fix what needs fixing and replace what needs replacing.
To me, this is just the other side of the coin of the in game performance situation. It appears as an outsider looking in to be an unwillingness to do what needs to be done to resolve the problem. Just to draw some contrast here, Everquest is set to release its 31st expansion this coming December which, I can guarantee, won't be just one raid and one wilderness zone. Granted, DDO quests are far more advanced than your standard go kill x of y mob and come back for a reward, but DDO expansions are still rather light when compared to other games in the genre at the same price point. And, just to rub salt in the wound, Everquest's 70 dollar collectors edition won't be required to play with all the new AAs and abilities instead of having to wait six months. Nor will you actually lose some of your character power during that period if you don't pay up... But that's another discussion so I'll bring it back on point.
Just like hardware maintenance, when it comes to continued development, the architecture of Everquest and the other games on the list who are still actively developing content for their customers doesn't seem to be a problem. To be fair, not all of them are still actively developing. FFXI, for example, has zero plans to release new expansions because they feel they have reached their limits. But look at all they were able to accomplish while managing to not have a laggy mess of a game and their maintenance windows are less than 2 hours. Take a cue from Square-Enix. If your game can't handle more expansions, stop making them.
TLDR: Less excuses, more results.