I think the issue here is the upgrade chain, which can get quite convoluted for Android. As you prpbably know, Google release the primary software, then the manufacturers adapt it for their devices (and may not do so for older ones) and then the networks do the same (and may not do so for older devices, or ones where their user base is small). And that last link in the chain is where the major logjam occurs.
It's not a satisfactory situation, and I'd like to see Google adopt a more Apple-like stance, with the core of the OS going to everyone and any manufacturer but network tweaks (which would still be allowed) are effectively skinning. That way, everyone could have updates and pretty much as soon as Google release them.
If you're not getting Marshmallow for your phone now, I very much doubt that you ever will, I'm afraid.
The solution is to buy SIM free so that you don't get the network version of the firmware and only have to rely on what the manufacturer puts out. That means either buying outright or going via Carphone Warehouse or another 3rd party supplier who sells SIM free devices.
