I've seen no such statement of Satoshi's. Please provide a link.
Piling every proof-of-work quorum system in the world into one dataset doesn't scale.
Bitcoin and BitDNS can be used separately. Users shouldn't have to download all of both to use one or the other. BitDNS users may not want to download everything the next several unrelated networks decide to pile in either.
The networks need to have separate fates. BitDNS users might be completely liberal about adding any large data features since relatively few domain registrars are needed, while Bitcoin users might get increasingly tyrannical about limiting the size of the chain so it's easy for lots of users and small devices.
Fears about securely buying domains with Bitcoins are a red herring. It's easy to trade Bitcoins for other non-repudiable commodities.
If you're still worried about it, it's cryptographically possible to make a risk free trade. The two parties would set up transactions on both sides such that when they both sign the transactions, the second signer's signature triggers the release of both. The second signer can't release one without releasing the other.
Of course we can't know exactly what satoshi had in mind, but that is irrelevant. What we know today is: We don't need to risk a hardfork in 3 months, there's enough space with 1MB (and now with segwit) to roll with it for at least a safe year while additional research is made on that possible hardfork to 2MB. Doing it in 3 months is just stupid. Why would you risk the current uptrend with another massive crash again due hardfork drama when we don't need to? Let's do things right for once.