I am not here to tout my own success, but considering I have been to the finals in 11 of 21 seasons playing this game, I can confidently state you can not only prioritize winning, but actually win while training players.
I think it becomes too expensive to train out of position in D1, but I agree in general with this statement (I trained out of position 1 week out of 4 so far). The game allows for tactical arrangements so the impact can be minimised. For example, if you train out of position at PG, you may play 2 guards at SG and SF and set them to defend PG and SG; and you can avoid tactics heavily reliant on the skills your trainees lack, so for trainees without PA and JR, it might be wise to drop PTB and LI in favour of LP, Patient or Princeton. Coincidentally these last tactics should also reduce the probability of injuries and the chance the trainees are subbed out due to stamina reasons.
That said, I still maintain that investing in players may still be more profitable than the extra revenues coming from investing in the arena. The risk is higher for sure, but the way things are going on the TL it's definitely possible.
Say you have 500k available in the bank. You can build seats or buy a player, the right player which is undervalued right now. Say you get the pricing adjustment right and the expansion will net you 15k extra per week (being generous, I think it's reasonable to expect 10k-15k once you factor in the drop in seat pricing), over 10 weeks that's 150k. In the same amount of time if you can flip a 20k player you bought under market value for 500k and sell him for 1m after 10 weeks, you will make roughly 850k from the sale less the salary you've paid (20k*10=200k). So that's also a 150k profit in the simplest of terms, however presumably that 20k player (worth 1m) would have helped you achieve a better record and gain better gates+merchandising in the 10 weeks you owned him. Investing in players is risky because there is no guarantee you can resell for 1m, it's made less profitable by taxes and it only works if the salary of the player is reasonably low. However the market inflation is your friend here and it's fairly common to see older players selling for more than their previous transactions, while the skills are the same or worse than before.