I think the mistake is to have unlimited messages if messages are such a major cost factor. Most services with "unlimited X" have a soft and hard limit on X if you dig deeper. If you don't want to charge per usage (or for overages), you should do a mixed calculation based on typical and min/max usage per customer and how usage ebbs and flows over time.

For the grandfather plan I'd aim lower than that even if it means losing a little money on high use customers because their usage might decline in the future (at which point they're providing profit while being less likely to cancel because of FOMO) and because the lower price point and legacy will make them good multipliers, not just for direct referrals but also as references or partners for whitepapers.

Basically you can get more out of them than just money so treating them the same as later customers means missing out on that.

I wasn’t, and don’t think other’s were advocating losing money on the originals.

Making them angry when they were on your side and helped you off the ground is a silly policy. The negative word of mouth factor alone could be incalculable and do unbounded harm, all for ~$100 bucks a month?

Treat them right and word of mouth will make much more than that in the long run.