Eberron originally came in a boxed set about Planescapes in the 90s, published by TSR. It is all about travelling between the planes. Astral castles fits the concept perfectly. Gamers, who were not OTB D&D gamers, are not familiar with the earlier products or precedents. If you are not familiar with the handwritten/published material, then, maybe, you should familiarise yourself with it.I mean they did, but they were very unlike the Age of Sail pirates we usually think of
That being said, while I like the aesthetic grab-bag that DDO's format allows it, I wouldn't mind circling back to a more High Fantasy concept for a release or two either. Fey was, but it was so whimsical
I don't think astral castles fit into the lore though, and couldn't just be swapped in for guild ships functionality
WotC and SSG went with Eberron and planescapes because they forecasted a future ability to include whatever expansion packs they wanted and/or could make them money. Their goal is to make money. They welcome suggestions because they are looking for new ways to make money. Leaving the game static for a long period of time will not make them more money. They want to grow their product and expand their fanbase, thereby, making more money.