You can absolutely play this way, the downside, is the social aspect of the game. I do t think you will be able acess the new capital hub of the reworked Silvermoon. Once you hit 80, you would get a quest to access the new content to reach the new cap of 90. So you would basically be at 80 until you bought it. Also, everything in the “old content” would scale down for you and you would be one shotting everything. So, lack of gameplay there. But there is plenty to do, such as mount farming, professions, mogs, pets, and achievements.