I believe that to be really good – nay, great – at music takes time, energy and persistence. The unfortunate thing about a “day-job” is that it compromises the first two.
If the day-job is in any way demanding physically and / or mentally, then there is only so much energy left at the end of the day and at weekends for music. There are brave souls who hold down a day-job, record, regularly gig in the evening and promote their stuff but I humbly suggest it would take a rare animal to excel at all of these.
In terms of time, living by music is not just about the music, whether it be composing, recording or performing. As I said, It’s also about promotion, networking, travelling, and – if you make money – accounting. That takes a bunch of time.
That all said, I have a day-job and I do what I can in music without setting goals which I don’t think are practicable to achieve with given the amount of work I’m prepared to put in and still have a life.