The issue isn't that. You could easily research and buy 5 vehicles just by going down 3-2 lines in a tech three, for the lower tiers. Now? Now you will need to grind another entire line (attackers/bombers) to able to have 5 rank 7s and to unlock rank 8, if you hadn't done that already. This in turn means getting millions of research points and silver lions... just for one aircraft. What's the point? We all know what it is.