The bug limiting maximum VLANs number achievable pesist in V5beta , too.
A router with more than 1000 VLANs defined is no longer accesible over network after reboot
In v5beta2 too you say? What other versions does this bug exist in?
its bug from early version 3.xx, if you have many vlan (> 1000) after reboot some vlan not running, its show in winbox, but without R flag. After disable and then enable those vlans - all works fine.
i simply cannot begin to comprehend why ANYBODY in ANY situation would put 1000 vlan interfaces on a single routerboard… it blows my mind 
there is no single use-case that i can come up with that needs 1000 vlans other than being an immensly large international backhaul carrier.
BRAS-type concentrator? I’m going to implement that myself soon.
Terminating wholesale L2 DSL services…
That sounds like an interesting to handle wholesale DSL…
The way I know from our home market is L2TP for run-off-the-mill DSL services or getting VLANs based on regions/cities, rather than per-customer.
However I do know that eg. in the czech republic symmetric DSL lines are handed over on a per-customer vlan base.
Also for a BRAS from what I experience in our home market only a handful of VLANs are necessary, not hundreds or thousands.
But, point made, DSL termination does seem to be vastly different in other countrys
terminating more than 5000 users, each user on its own Vlan
An even better aproach ( for performance and management ease in BRAS scenario) would be to have the capability to offer services like PPPoE simultaneusly over any and all possible incoming VLAN.
The only real use for the VLAN tag in such a scenario will be to be handed to the Radius server for authentication and accounting.
As far as I can tell CISCO BRAS can be configured that way.
Since all kernel VLAN logic can be bypassed in this scenario better performance cam be theoretically be achieved.
BTW, it could be useful for PPPoE AC to be able to reply for any PADI request, regardeless of the server and service name. CISCO offers that option as well.
Another nice feature would be to be able to have different MACs on different VLANs ( maybe by generating fake MACs with VLAN number in some positions). Could help if some VLANs get inadvertently wired together in the network.