But usually once the vendor stops supporting the gear, its usefulness is gone anyway, sure, you can load openwrt up on many cheap single band 2.4GHz routers that are 5+ years old, but so what? They have weak CPU, often only single or 2 chain 802.11n support etc.
Last I looked, noone had a decent way of having openwrt devices pull configs down from a central place to configure them up, with the ability to automatically do wireless uplink and provision devices that were not physically on the same wired network like the unifi gear does, and you are not going to get anything comparable to the ac lite range for the same price when you actually pay for your time messing about with open source things that dont work as expected because the documentation is out of date etc.
If you were putting 100s of things out there then the lower cost hardware might make sense. For a one/two off thing, it doesnt. Plus when you are gone this place will have to find someone else to come along and support it. Setting them up with something custom based on open source is just setting them up with a giant load of hassle in the future.