"Ulduar is longer and harder, so what"? If you ignore the rhetorical prose, SilverFlash poses an interesting question. I do not think anyone is claiming that releasing Ulduar will eliminate downtime. I believe the point Pendrick and other are making is that 1. It will take longer (more hours/raid days) to clear Ulduar and 2. There will be additional challenges for a significantly longer period of time.
To expand on point 2, how many weeks did it take for 75% of raiding guilds to clear naxx 25? SilverFlash, how many weeks do you think it will take for 75% of raiding guilds to clear Ulduar 25 + hard modes? If the number of weeks is significantly longer for the latter, what is the logic for keeping content the same amount of time? Personally, I would like to see my (casual) guild have a shot at some of the hard modes and maybe even see Algalon. However, in order for more casual guilds to have a shot at clearing Ulduar content, it has to be out for a longer period of time relative to Naxx 25.
A guild that has cleared all available content has little motivation to log on except to level alts. A guild that still has content needing to be cleared will have goals to work towards. If there are still bosses and hard modes to overcome, people are incentivized to show up to raid, acquire better gear, form more 10 man pugs to practice mechanics, etc.
"Two weeks in and somebody will just bring the topic up again". While that might be true, I think it is important to consider percentages. I agree that someone will likely bring the topic up again (a top 5% player in a hard-core guild). But how many people will agree with them? If the majority of the community wants to move forward because they have exhausted the current content, why prevent that?