Good article from Peter Bolt singing the praises of full-time theological study with many sound pragmatic arguments.
You cannot easily proof-text a reason for this (beware those whose only diet is proof-texts).
The theological reason for full time theological study is NOT found in the Bible. It is a wisdom issue.
My 2c: you need 3-4 years full time study to catch up on what the ascended Lord Jesus Christ has been doing for 2000 years, in light of the Old Testament unfolding vision culminating in His New Testament death and resurrection – in the original languages. It’s as much in the integration as it is in the individual subjects.
Why would you want to change that wonderful inheritance from the Lord that has worked well in the past and is still working well and will certainly keep on working well into the future? You would have to be brain-dead, perhaps? But that is probably a rude thing to say, so I withdraw the remark. Certainly better reasons than I have heard will need to be advanced before I am convinced that full-time theological education needs to be ditched from ministerial training.