Only because the community is so small that telling "everyone" to downgrade to 0.7 is actually doable. Now imagine a future where everyone uses bitcoin - imagine getting an institution like a bank to get on the "right" version...

Actually, for this purpose the system is not getting any bigger right now.

You only need to notify miners and mining pools. These have been concentrating for a while and will continue to concentrate in the future.

Individual users who aren't mining don't need to do anything.

While dropping to v0.7 was a method to resolve this in a manner which protected v0.7 users it wasn't necessary. Miners could have continued to mine v0.8 chain. Transactions from the v0.7 blocks would be included in v0.8 blocks.

Users could be warned to STOP using v0.7 and upgrade to v0.8. The advantage of downgrading it is resolve the situation quicker. Far easier to get a half dozen mining pools to switch to v0.7 then get thousands (tens of thousands) of users to upgrade to v0.8. A v0.7 user who followed the warning and stopped engaging in transactions either until v0.7 was the longest chain or until they upgraded to v0.8 was not in any danger. Existing transactions flowed across both halves of the split.

They'll need better systems to deal with it.

Like every other system, including the existing ones, it'll have to get better as it scales.

They didn't tell "everyone" just the people that are running mining operations.