If you can't get 50 wins in a row twice you get the Bronze achievement eventually. Or you do it again.
Firstly, stacking is a good idea too. Putting that aside: Let's take player X and player Y. Player X got 3 crits in a battle, while player Y constantly outplays his opponent. By achievements we'd rank them the same. The cool thing about fading achievements is that they show off how often you're getting these achievements.
It's not a ridiculous fear. The whole point of achievements is showing off skill. Getting lucky once kind of goes against the whole point. If you got it a while ago, you still have the Bronze achievement or whatever.
A chore to get? You can brag to your friends about them... before they fade. And you don't exactly need to get them. If you think it's a bother to get them, then don't. You will always have proof that you did it if you want.
So here's my proposal:Stacking AND fading. I think stacking>fading somewhat, but I like the idea of both. That way, you can show off how many times you've done it, and combine with how recently you've done it, too. Ideally when you mouse over...whatever visual representation it'd say the last time. Hopefully it would expire based on logged-in time, but I don't know if that is really implementable or if it's worth it.
Also, if fading exists, there really need to be two tiers. Easy tier: 6-0's, for example. They go to silver in a day and bronze in a week. Hard tier: Topping the ladder. Silver in a week and bronze in a day. if the logged-in time ever gets implemented, we'd need to change the timing, but I don't know if that can even be a reality.
Edit: Messed up? You'd still have them in a lesser form. As I recall, it's probably going to be Bronze, Silver, Gold.
Yes everyone gets lucky, but the point is to show players that can consistently get these achievements. it cuts both ways? Yes, but the thing is... you want to minimize the impact. If you just got a 6-0 once and that's it, it's not a great measure of your skill. Even broken clocks are right twice a day.
And why would it be a pain to get them again? Again, if it's a pain then don't DO it. You shouldn't be putting much extra effort into these achievements anyhow. Most of this stuff is stuff you should be trying to do naturally. Win a lot? Win dominatingly? Even the comeback can be good strategy-sacrifice your pokemon to ensure a sweep by your last one.