>>11839138>The ones in OU are offensive threatsNot really, no. I guess most of them can function as offensive Pokemon, but a lot of them are generally used in defensive roles.
I took the time to break them down.
(d) Celebi - used almost exclusively in defensive roles
(d) Heatran - usually used as a specially defensive tank
(d) Jirachi - premier special wall, tournament staple
(o) Keldeo - offensive
(o) Kyurem-B - offensive unless you count SubDT as defensive set
(o) Landorus-I - offensive
(d) Landorus-T - offensive potential but generally plays defensive roles
(-) Latias - usually used over Latios for its defensive qualities, not sure if its usual role can be described as "defensive" though
(o) Latios - offensive
(o) Terrakion - offensive
(o) Thundurus-T - offensive
You'll notice that this is a fairly even split (4-6 excluding Latias). Feel free to see what happens when you exclude the ones introduced in gen 5.
The rest of your post once again refers to Pokemon outclassing others. I agree that having a single best option for any given role is a bad thing, but this isn't a new phenomenon that got introduced alongside legendaries. It has always been around. It isn't significantly worse in gen 5 than it was in previous generations.