Short version: please keep Classic for NAV7. Just for old clients, no need to sell new licences of it.
Right, of course the new customers won't need it. But you have customers who are using NAV since 10-15 years, have a metric ton of developments made. Given that everybody knows your support strategy is 2 major versions back and NAV6Classic will be supported until NAV8, they could decide to upgrade to RTC during NAV6 or wait for NAV7. Waiting for NAV7 might sound a bit brash but not an irresponsible decision since NAV6Classic will be supported until NAV8 comes out, so at least 2 years after NAV7. The advantage of upgrading to NAV6RTC would have been gradual upgrade, one role at a time. The advantage of waiting until NAV7 is the non-crazy report designer i.e. actually handling data hierarchies.
The best solution for everybody would be the ability to do a gradual RTC upgrade with NAV7. Hence the continued support of Classic would be necessary.
It won't cost you much. You need to support NAV6Classic anyway, so you just set a bit to let that client work with a NAV7 database, even if we lose some new menu items or fields introduced in NAV7 or suchlike it is not big deal, we can customize that back if needed.