If you buy them new then you have 8 years before D-checks come due, so that's 8 years to build up enough cash to get an extra plane or two. And the likelihood of you only flying 7 of them after 8 years is low. Now, if you have only seven 773s but a larger fleet of 772s, you can substitute the smaller planes for the duration of the D-checks. Alternatively, if you have only 7 of the "ER/LR" version and your shorter-range 777s can't handle the ER flights, you could lease one or two planes short-term to cover the D-checks and start them early. You don't have to wait for the D-check to expire, you can do it early. My rule of thumb is that if the check expires within the duration of the check (i.e. within 60 days for a 737/A320) then I will do a check early if necessary to avoid a maintenance crisis later.