What a beautiful lemon tree! Even without lemons, I wouldnâ€™t get rid of it! However, if you want to encourage fruit, then (as someone else suggested), Iâ€™d try transplanting it. As to the lichen, I don't think thatâ€™s a problem. Mites however (if theyâ€™re present as someone else suggested they might be), could be a big problem if not dealt with. Iâ€™ve had lemons on my indoor lemon tree, but only a couple over several years, and not of high quality. So perhaps, potted lemon trees just struggle more when it comes to fruiting. Good luck!