So, you are right. The Gold is a major piece, at least on normal boards.
OK, so we agree that the only issue is fortress draws, where you can avoid loss of the piece indefinitely, but cannot make any progress towards a win. The Woody Rook (WD) has that problem:
In this position it is 'dynamically trapped' in the corner by the bare King, which will approach along the diagonal and eventually capture it when it stays stationary. And it can stay on the same diagonal as the WD (without approaching) when the latter moves, to renew the threat. Yet 96.9% of all positions with the strong side to move is a forced win. (The Checkmating Applet reports 70.6% wins when the bare King has the first move, but the remaining 29% includes all positions where that first move captures a King or unprotected Woody Rook.) So althoug the possibility to hang on to the WD doesn't guarantee a win, it is still useful to think of the WD as a major piece, as it is really exceptional that it cannot force mate.
This would be quite different for the Evil Wolf from the large Shogi variants, which is a Gold without the backward step (fsWfF). If the bare King can get behind that, it is safe. Still 16.7% of the positions where the bare King has the move are forced wins. In practical games the bare King would likely be close to its own back rank, and you would avoid frivolously advancing an unidirectional piece like the Evil Wolf, and tend to assign it a defensive function in your own camp. So in practice K + EW might still almost always be a forced win. In any case it would be quite wrong to think of the Evil Wolf as a minor.
Although a Silver can force mate in 4 in some postions, these make up less than 0.1%. So considering the Silver a minor seems quite justified.
OK, so we agree that the only issue is fortress draws, where you can avoid loss of the piece indefinitely, but cannot make any progress towards a win. The Woody Rook (WD) has that problem:
In this position it is 'dynamically trapped' in the corner by the bare King, which will approach along the diagonal and eventually capture it when it stays stationary. And it can stay on the same diagonal as the WD (without approaching) when the latter moves, to renew the threat. Yet 96.9% of all positions with the strong side to move is a forced win. (The Checkmating Applet reports 70.6% wins when the bare King has the first move, but the remaining 29% includes all positions where that first move captures a King or unprotected Woody Rook.) So althoug the possibility to hang on to the WD doesn't guarantee a win, it is still useful to think of the WD as a major piece, as it is really exceptional that it cannot force mate.
This would be quite different for the Evil Wolf from the large Shogi variants, which is a Gold without the backward step (fsWfF). If the bare King can get behind that, it is safe. Still 16.7% of the positions where the bare King has the move are forced wins. In practical games the bare King would likely be close to its own back rank, and you would avoid frivolously advancing an unidirectional piece like the Evil Wolf, and tend to assign it a defensive function in your own camp. So in practice K + EW might still almost always be a forced win. In any case it would be quite wrong to think of the Evil Wolf as a minor.
Although a Silver can force mate in 4 in some postions, these make up less than 0.1%. So considering the Silver a minor seems quite justified.