Nah, the milestones are easy enough to get as it is and in the case of the Spy and Sniper are actually EASIER to achieve than waiting for the unlocks.
Apparently the average time needed to get an item (considering 25 minute checks with increasing probability) is 1 hour 40 minutes - and you don't know if you won't get a duplicate. And you get items from other classes as well.
Because you have no difficulty in obtaining the achievemnts of the other classes, some, like me, do have difficulties.
Medic and Pyro achievements, I have enough for all weapons. However for Heavy I have not enough and for scout I have absolutely none. Simply because of the time I do stick into them is not enough to get an achievement. And although some can be obtained by playing the class a lot, in my 'world' it would simply mean that with achievements, I would not get both the heavy/spy/scout unlocks in the next years.