Full disclosure: lead architect of OCIS here.
OCIS is currently targeting the file sync and share functionality only. The idea is to leverage the microservice architecture to scale individual services as needed. In the beginning, we cannot bring back all features of oc10.
Personally, I am looking into kopano as a mail and calendar solution. We already embed konnectd as the OpenID Connect identity provider and it might make more sense to work an integration the rewriting the calendar and contacts.
There are always other ways forward, however.
Aside from integrating better with kopano we could look into roadrunner, a go implementation of pho from that allows handing off requests from go to php. It would be possible to implement the oc10 php interfaces and run existing oc apps in an independent sandbox as a service.
But I don’t see that happening until we, as in ownCloud GmbH, have a solid product feature set for an enterprise file sync and share solution. Do one thing and do it well.
I am super happy the Klaas is back on board as he knows a lot about the desktop client and the sync protocol. Which, in the end is more important than the technology stack being used.
That being said you may have noticed the lack of new features in oc10 over the past year. We were focusing on OCIS, but are investing more people in oc 10 as well. It will get an update to php 8 and we are investigating ways of running OCIS or at least some services along oc10. The OpenID IdP for example will become more important as it allows properly authenticating clients.
In the end it might be a longer transition phase. Please let us know your thoughts and concerns.
Again, personally I am looking into kopano for calendar and contacts as it allows using active sync to access it from any Android device without installing an additional app.
If anyone wants to dig into golang and roadrunner ping me in https://talk.owncloud.com. Happy to help get started! See https://roadrunner.dev/
So… 2021 will be an exciting year for ownCloud!