If you actually used containers in production, you would know what we are talking about. While you might have unconditional love towards Proxmox 4 and LXC, and want to get rid of all "old technology" as soon as possible, the reality is far more complicated than that. Many organisations actually depend on OpenVZ, and upgrading to LXC is far from being straightforward.OpenVZ is a dead project, only maintained in its current state locked to old kernels with old technology. I'm glad Proxmox decided to keep up moving towards the future instead of keeping us locked to the past.
Eventually you will need to stop using OpenVZ and migrate to something else. Even if you stick with virtuozzo 7, the successor of OpenVZ, you still have to migrate away from OpenVZ. https://openvz.org/Upgrade_script_from_OpenVZ_to_Virtuozzo_7
It's more like letting you choose to run Windows XP (Proxmox 3.x) or Windows 7 ( Proxmox 4.x)
LXC on Proxmox 4 has very serious shortcomings right now, which make it unusable for us (and many others):
- there is no snapshot backup capability (on LVM), so downtime is inevitable
- there is no online (or at least suspend) migration ability
- the fact that /proc and dmesg show host info in the guest is a catastrophe for any kind of secure or untrusted environment (like hosting)
- there are serious performance and stability issues
Furthermore your Windows XP and 7 analogy is misleading and incorrect: Windows 7 was feature complete on release, while Proxmox 4 is (at the moment) very far from that.
So I think I have to join others here who ask for continued support of Proxmox 3.4, at least until LXC reaches feature parity with OpenVZ... Ceasing to support Proxmox 3.4 in 3 months is going to leave many users (us for sure) in the dark, as we believe LXC on Proxmox will need another year to reach maturity - and only if the Proxmox devs agree with us that the above listed issues need solving. If not, than we might never upgrade to PVE 4.x...