Why not just add object orient support, and keep the old data store format and transfer protocol, so customer could use MT5 client to receive data from MT4 server?

Some things like free period support such as PERIOD_M2, PERIOD_M6..., is useless except PERIOD_H8, PERIOD_H12, because period must use 3 or 4 as multiplicator. It's unnecessary to support so much period.

To keep compatibility with old MT4 server is very important. I think the broker maybe not interest for the MT4 to MT5 upgrade, because it can't make more profit, just consider that, customer will not change their trade platform only for the new MT5 platform upgrade. If the customer feel the MT5 platform is necessary, she or he should be like to pay for it. So for bussiness consider, if MT company want to make money from MT5 product, it should consider make profit from end user, not the broker user.

The MT5 product is change so much, it's most likely get into scrapes that most broker users and old MT4 end users are not willing to do the upgrade. Why not just keep the MT5 back end compatible to MT4, if the end user realy like the MT5, let them pay for it?