Interesting.
I've always wondered if your solution was better, or if Mizu's one was superior (an extremely strong starting 5 and then only 1 backup). Mizu's solution is vulnerable to injuries, while yours will struggle against Mizu's, as your starting 5 won't even match up his.
But 15 players is way too much. Sometimes it helps to have a shitty player in decent form, and you can't have this with 15 guys.
I went for a strong starting 5, 2 very nice backups, who wouldn't have any trouble being a starter if a starter is injured, 3 mediocre backups (doing their job in garbage time and easy cup games) and 2 shit players.
That's 12, and you pretty much cover everything might come, not to mention you're relatively immune to 1 injury. However, should 2 or 3 players get injured you're in deep trouble because the remaining players will play more than 70 thus see their form drop.
In this particular case, I buy 1 k player(s) to soak up extra minutes and wait for better times ...
However, try to train staring 5 or backups, not mediocre or shit ones. Because in this case you'll have to drop training way too often.