I am supportive of a subscription plan at a reasonable cost.
I will be interested in how you will handle the existing credits (we have a sizable balance and this was one of the appealing features of emonCMS - the more we buy from the shop, we get credits for emonCMS hosting).
Perhaps one way to keep purchases separate from subscription plans is to offer discounts on future purchases of gear from the shop vs subscription top up credits. The current emoncms subscription credit is 20% of the purchase invoice price. How about creating a shop voucher for future purchases?
Alternatively, provide tiered subscription rates based on the number of feeds. If I purchase more gear from the shop, I will create more feeds, so a discount for having more feeds is somewhat like the credits we get today.
Of course, all this depends on the subscription rates you plan to introduce. The current rate of 1GBP per year per feed is reasonable for a small number of feeds, but it gets expensive with larger numbers. With my credits and current number of feeds, I have covered over 5 years of emonCMS use. Changes to the current model will impact my budget going forward.