I wouldn't say "All DECT subscribers on the 4K are configured as UFIP rather than the old CMI. That't about the differences."
To use DECT IP your data network must meet all VOIP requirements, and not everybody's does (although they'll tell you it does). Sure you've got DECT to the base station, but then you're using VOIP on customer network to get that base station talking to the 4K. And that's a huge difference. You've now got two things to manage instead of one. User has a poor quality call? Data team say "PABX issue. Nothing wrong with the LAN!" etc etc. OSV has to implement DECT this way, 4K can also do it this way, but 4K can also provide TDM all the way back to 4K - rock solid. Only needs 1 pair to bring a base station in service. Add one or two more pairs for more capacity.
Cordless IP will meet some scenarios where LAN is already deployed, easier to just hop on the LAN rather than install new line & plant for sure. But if you don't want the hassle of VOIP muddying your DECT waters, stick with SLCs.
I cannot agree at all with your comment about scalability. It's more elegant? No chance. With DECT IP you need to know where you're going before you start. You have three options, small, server, or large system. Small system only supports 10 BS and 10 phone calls, with ~50? logged on phones. So move up to server based system. Now you've got a linux server in the mix (hope it doesn't crash), with up to 50 BS and 50 calls. Need more? Large system. 3000 BS max, but now you need a management server and up to 100 media servers. That's right, up to 100 media servers. Wow, what just happened? Should have gone pure DECT on 4K SLCs is what just happened.
4K scalability is much easier, Install 1 SLC, you get 24 BS ports, up to 16 individual BS, and ~120 handsets. Need more? Add another SLC. Need more? Add another SLC. Need more? Add another SLC. Oh God I'm bored, it's too easy, up to 48 SLC's per 4K, over 3000 handsets per 4K. It should be enough for anyone but if it's not, add another 4K, certainly won't take as much room as your media server marathon and 4K offers seamless handover between nodes with ISS. There are some monster 4K DECT installations across Europe, I would be very surprised if DECT IP had anything similarly sized. It may get there, but I'd feel sorry for anyone filling their comms room with media servers when they could have had it all internal on 4K SLC cards. Nice and easy maintenance too - from connection to 4K you can do the lot, all in one box.. no hassle with multiple external servers.
For sure, small branch office that needs a bit of cordless, DECT IP no problem. Campus wide and life & limb? Difficult sell if you're able to use 4K DECT.