I might be bias since I was fully homegrown for 4 years and even now just having one of my old draftees bought back in my team besides the ones I've trained all by myself.
For a homegrown team it definitely sucks that you have to kinda make due with what you get. For me it meant having my bigs be 6'4'', 6'10'' and 6'11'' and my guards be 6'0'', 6'3'', 6'4'', 6'7'', 6'9'' and 6'10''. You can probably imagine how difficult it has been to have any kind of coherent training to them since everyone develops in such different pace. With the suggested system I could've trained the 6'10''+ guys to be the bigs and then below that for guards. The team would definitely be more competitive at least. But then again I have been able to compete pretty well even with these circumstances.
However, there's absolutely no chance I could challenge the top teams of the country, so yeah while being homegrown is quite profitable, it's not a strategy that you can succeed with in the bigger countries even if you do everything right and get extremely lucky with the draftees. With the suggested change it might become little bit more viable strategy, but still I doubt we'll see a homegrown national champion from any medium to large country. That would require speeding up 2 position training to maybe 85-90% of one position training, or having some kind of homegrown team chemistry boost to the team performance. But that's whole another topic.
I also had another compromise option come in mind. Maybe we could implement the plan A, but the condition could be that you have to set the plan for next week before Friday training update. So if everything goes well and you follow the training plan, you can have training from multiple positions. However, screw something up and you can't change the plan. If you follow the old training system where you can train only one training, you would still be able to change the training mid week.
So you could choose to play it safe by the old system or risk it with the new system. It would make sense that more complicated training regime requires more planning beforehand and is more difficult to execute - therefore you'd have to set the schedule early. Though this might cause quite many angry posts to the forums when someone risks it with poor results :D
Anyway, I'd be fine with the current system, plan A or plan B. I know me and all other homegrown teams would benefit from the changes.