Using classic Outlook for Windows in business environments
The source code excuse makes no sense. If different branches of source code were such a problem, you'd want as many people to move across as possible, as quickly as possible.
Baking scam ads and a subscription into your new platform does not make people want to switch, it makes them want to stay on the old platform, and if you force the switch, it'll make them migrate to a competitor. Nobody wants an email client that delivers fake emails.