i use a liner only to keep the bag clean. i find that the temperature rating of liners are generally massively overstated. just imagine adding a layer of clothing that’s made from the same material as the liner and you will have a good idea of what it will bring to the party.
sleeping bags do become less effective the older they get and one has to be creative to prop up the sleeping temperature. pay attention to:
insulation from below (i.e. sleeping pad)
type of shelter (sharing a 4-season tent with a couple of homies will add a few degrees)
layers of clothing you can sleep in
One can also buy fleece material quite cheap and anybody with some skill with a sowing machine can make you a home made fleece inner - will be a bit heavy but does save a lot of bucks if you use it only on the odd occasion where you have to add a lot of heat