If MLB and the players union can’t reach an agreement to start the season, could the owners use all minor league players instead?
The first thing to note here is that there is no absolute need for an agreement to be reached - MLB has the ability to impose a schedule on the players without an agreement. Of course, if the agreement is too objectionable to the players, they would have the option of going on strike.
In the hypothetical situation of a players' strike, the team owners could in theory use minor league players. However, I suspect that neither would the owners actually do this - minor league players are not going to make a product that fans are going to tune into on television - or actually that they would be able to find enough players of even a reasonable standard to fill out the rosters; the experience of those players that crossed the picket line in the 1994-5 strike would put a lot of players off (which is of course the MLBPA's idea here).
I will dissent in part and agree in part with Philip's answer here.
It's possible the owners would be able to start the season without employing any players represented by MLBPA (which is in effect what you're asking). However, I think this would be exceedingly difficult. There is a difference between a strike and a lockout, and in the latter case (when the owners are refusing to permit the union members from working) they can't necessarily employ other workers. The 1994/1995 strike was a strike, an action on the players' part, and thus the replacement players were permitted to be hired on whatever basis they choose; in a lockout, they can only be hired on a temporary basis, and that may significantly complicate matters with minor league players some of whom would ordinarily have become players anyway - think Luis Robert for example (I'm actually not sure if he's a member of MLBPA at this point, but if he is, there are other similar players who certainly aren't). This ignores of course the optics of the situation, which would certainly not be favorable for the owners here.
Even if they did attempt this, it's possible they'd be required to pay the major league players. None of us have the legal document they signed on 3/26, so I don't think any of us can say exactly what would happen if they attempted to field games, but it's possible they agreed explicitly to pay the players - which might supersede their right to lock the players out. If they did explicitly promise to pay the players in that document, then they may be out the money either way - meaning the only way to avoid paying the players would be to either cancel the season, or start it under such terms that the players would choose to strike (in which case they, of course, don't get paid).
Finally, attempting to lock the MLPBA out and play with minor leaguers this season, only one season before the Collective Bargaining Agreement comes to an end, would be such an incredible act of self destruction that I can't imagine it would even come up in the Owner's meetings at all. We could end up with multiple years without MLBPA players playing baseball games, which in turn would allow the TV stations to end their contracts - costing the owners billions of dollars - and who knows what further damage.
As such, I'd say that for the common usage of "could", the answer is "no": just as no-one would state that I could become President of the US in 2021, so too would I say that the owners could not field teams of non-MLPBA members in 2020.