Asking these players to wait a few weeks while you try to form a 25m is often not realistic. Cross-realm 25m for current tier has its perks, but it almost guarantees no mid-low tier 25m guilds will be formed on any but the most populous servers. If they already can do it cross-realm, why form a guild for it? And why limit it to 25m? It is a dangerous road.
Why is guild access to content more important than individual player access to content? How is WoW gaining anything by forcing players into guilds to get access?
Players will continue to form raid guilds when they enjoy the company of those they raid with. But too many realms don't have the population to support more than a tiny number of active guilds - which leaves a lot of players out in the cold when it comes to raiding. Cross realm raiding of current tier will help with that. No one is going to be satisfied with waiting until the content is old to get a chance to do it, and they're going to even less interested in doing the content another 2-3 months after being stuck doing that content in LFR for 5-6 months, especially when LFR is dropping better gear.
If players end up leaving their current raid guilds and moving to cross realm raid groups, it's because they were only using the guild as a way to get a raid spot and not because they enjoyed being part of the guild or were in any way loyal to it. At the same time, those guilds now would have a chance to recruit cross realm for other players to join their raid group so their players will continue to get a chance to raid.
Blizzard could easily allow raid team names to be set if people are worried about progression rankings, like Arena teams have names. Cross realm groups would be ranked by region/world. Guilds could continue to be ranked by realms given a certain number of players from the same guild in the group, probably 80% like Blizzard sets for other group content.
But the important thing is that more players would have the opportunity to actually raid instead of getting dumped in LFR for lack of options. More raiders in the raiding community strengthens the community and can actually lead to 25man raiding reviving since the 25man groups will have far more players to attract then the limited pools on their own servers. DPS would have a much easier time finding groups since the percentage of DPS in 25man groups is higher than in 10man.