I’d be more in favour of a warship style system where you can train a (limited) reserve.
Whilst in theory it could hurt Subscriptions, I think it would more likely encourage take up. It would also encourage different troop combos.
I know it sounds weird to say it could work to encourage it, but once you have ER... it is tough to go back. The ability to switch troops on a very limited basis is a bit of a taster of what it is.
Subscriptions for instant troops appeals to different player bases.
For those pushing the LB’s (of which there are many) the Subscription is much cheaper than it was to pay for diamonds and there is no need to save them over the months.
For those who aren’t doing any of those things and are more casual players, is there any need to do more than get your daily chest, events, VP chest ? Ignoring warships for now lol.
But there are a core of people, in between those two extremes that could be tempted to take it out... but they need to see the advantage first...