I never played Eve so I'm speaking from ignorance here, but there's two basic problems with this:
1. The attackers have nothing to lose by attacking and no investment to trigger the vulnerability. This is a critical point because if the attackers don't have to pay anything to trigger a siege there's no consequence for losing. In Shadowbane I remember the bane stone being somewhat expensive (at the start anywya, maybe it got cheaper as time went on). In ArcheAge you have to bid for a scroll against other players.
2. Are you able to move your stone out of your bank to trigger the vulnerability period?
2nd question first: once your structure is reinforced and invulnerable, you can't move stone to change the timer. It becomes vulnerable to destruction once all the stone you had in their is consumed, which is 0-36 hours depending on how much stone you had in there. If you are online while you are being attacked but before it goes invulnerable, you can adjust the stone levels to change the time, but once you are invulnerable the timer starts and it can't be changed.
as stated in eve and allegedly in Crowfall too you lose shit dying so they do have something to lose by attacking. It really depends if they have siege weapons in crowfall or not on how much you have to lose. In Eve you can grab a dozen or so dudes and attack a unmanned tower in battleships (IE your warrior type), assuming the tower has no defenses or you can mitigate them (this would be like NPC archers on the walls shooting at attackers), but if you all you bring is a bunch of battleships it will take like 6-12 hours to do enough damage to bring it to reinforced (ie drop its shields down to 25% health) and make it go invulnerable. If 1 player actually shows up during that time frame they can man the guns (or in CF case they could direct the NPC archers to focus fire on individual players) and just pick off the attackers while safely atop their wall.
To actually legitimately attack something requires you to commit massive resources on the field and that is a huge risk. I know CF teaser videos shows dudes knocking down walls with a single axe throw and shit but they've stated that that was just a voxel test and that players wouldn't be able to do that, except for certain classes like the guinea pig engineer dudes or whatever they are, who only build and knockdown things.
They may not do Siege weapons (KS budget after all), so you may not have the risk of Eve but this game is Eve-Lite anyway, much more SB 2 than Eve meets Game of Thrones. But invincible keeps are bullshit. I should be able to attack you whenever I want. If keeps are only vulnerable for a small window (per day? per week? how long is your gay ass keep going to be invincible for?) then defenders have a massive advantage.
The purpose of any of these systems as described by Todd is that the two groups get together and decide upon a time when they can fight. The defender already has every advantage going in, even larger if the defenders are the ones who choose when they become vulnerable. Oh twice a week I have to allow the peasants an opportunity to attack my keep? Ok Friday and saturday night during my primetime is when my keep is attackable. all other times fuck off.
It's like they saw what happened in Shadowbane with their invulnerable keep system (oh but allow teams with massive resources the ability to break through with banestones), and built the game around that. It seems like they took their flawed system and made it worse to speed up on how long it takes for the winning guild to win, so they can reset the campaign world which is their major mechanic. Why bother with 1-6 month campaigns if you are just going to speed up the rate in which the snowball is rolling downhill? 1-6 week campaigns would work.