Well, that attack isn't trivial. It would be incredibly difficult to spin up 100 relays without getting noticed. If LEA are running relays, it's most likely fewer than 10. Also, watching certain sites, like Google and Facebook, would be really hard, because they use distributed content delivery networks. A user in Seattle accesses a different server than a user in New York when they go to the same site. It might be useful for specific smaller sites, but if the attacker runs 10 relays, then the chances of picking 1 of them as 1 of your 3 entry guards is 1/30, or only 3%. And if you don't pick any of them, the attacker would have to wait 1-2 months for you to new pick news, with again only a 3% chance of pwning you. It's not an effective attack for identifying a specific person. More like, "among all the users of this site, I can find a few of them every few months". Also, none of this applies to hidden services, since the attacker can't watch the other end, although there are different potential attacks there.