Its completely different. Yes he's a 20 year old, but he's a 20 year old that many clubs believe is ready to be a starter right now. This is also his 3rd year in first grade.
Hes basically been offered a promotion (starting role) and more money elsewhere. Doesn't matter if you're a footballer, 90% of the population would do the same in their work role.
If he were to leave right now, he would start at a few teams. If he were to stay, he would have to wait until 2027 at the earliest to get a starting role, probably 2028. Thats a long time to wait and a lot of money lost.
What needs to be taken into account is that he would lose a heap of money staying in a back up role as opposed to a starting role. Not only would he get more money immediately, his next contract (in 3-4 years time) would be based on already being a starter for 2-3+ years. If he stayed a back-up, he would get less during that time and even his next contract would be less as he would not have proven himself to be a starter. Staying would impact his long term earning capacity. Galvin was starting week in week out and was getting big money offers to stay or to leave.
I would be devastated if TDS left, but I would completely understand and would not have any negative feelings towards him. He seems like a loyal kid who wants to be here but is unfortunately stuck behind Api.