An early version of the new Outlook on the web will be available through an opt-in toggle

Is it the intention that this 'new' outlook experience will become the default Outlook Web experience, or will it remain an option ? 

I have to say I am NOT a fan of the 'new Outlook' exoerience !  It seems retrograde in look / feel,  looking backward to the fat installed client instead of the light open feel of the current Outlook Web.  Missing features which immediately confronted me were:

No 'delayed send', the 15 second countdown.   It's in the current web client, why on earth remove it if it's not a UI issue ?

Groups display no longer shows the group's picture / avatar.  I'm a member of a lot of groups.  Taking away that simple visual indicator and reducing it to a list of text slows me down considerably in navigating my groups.

Folder lists are now more distracting with the compacted layout with bold (subject) and blue coloured recipients.  That's the experience I finally got away from when I stopped using the fat desktop client !

Overall it seems like useful features and settings have been removed, perhaps to simplify things for MS, but at the expense of usability. The UI has been dumbed down to be closer in look/feel to the fat client. 

When Google introduced Inbox  I went over to that for a while, but pretty soon came back to Gmail for very much the same reasons of preference for a cleaner UI with no features that attempt to manage my email for me in a 'smart' (ie. I get less control) way.

At best please continue to give us a choice now that many of us here in my organisation have adopted Outlook web as our sole email client, not least on the basis of the cleaner UI.