I also receive pdfs via email, but I don’t want a pdf reader in my email client. It’s perfectly feasible to write an email client that hands off received event

I also receive pdfs via email, but I don’t want a pdf reader in my email client. It’s perfectly feasible to write an email client that hands off received events to a calendar application and receives the response to send back. It’s all icalendar components under the hood anyway. Putting everything together in a single monolithic program that handles email, calendar and contacts is a design choice, not a technical requirement. And yes, I do use the workflow that your descube receiving and sending calendar events. But Thunderbird’s design is annoying because it’s a all or nothing situation: you can’t just use it as an email client and use some other calendar application.