Forget about the 2006 forum post. A lot has changed since then version 0.1 of Roundcube was not released until 2008!
Pressing tab has always caused field focus to change, it is the expected behaviour. The post from 2006 was about changing the tab order to make keyboard navigation easier it did not intrudce any new behaviour.
Also I notice that the tab order is differnt in different skins. In Larry for example it moves to the attachment panel before the send button so may be try that and see if you like it better. You could also tackle it from the skin side, extend the skin you are using and replace the compose template with one which uses a different tab order.
Ultimately Roundcube is an open source project it relies on people giving up their time to contribute. People naturally like to work on the more interesting and popular ideas. Plugins can be a good way to really gauge the usefulness of new ideas, we've seen popular plugins being merged into the core.
No one is saying what you want cannot be done its only that the experience so far is there is no big demand for the sort of change you are talking about while there is more feedback about easy keyboard navigation. So there is no apparent need for a change in the core.