There are several facets of the construction rules that seriously disfavor Light 'Mech compared to Vees, here's a quote by Moonsword that illustrates this:
You do understand how vehicular facings work, right? Vees need more armor than an equivalent mech just because of how much firepower tends to bunch up on a single location or two.
And sure, the J. Edgar (if you want to use that as an example) can move faster for cheaper, but also gets massively shut down the moment woods, hills, LB-Xs and the like come into the equation, And all it takes is one bad crit for that vee to suddenly become immobile and get taken out that much faster. So yeah, they might have some advantages over light mechs, but you really can't say that they render light mechs obsolete. At all.
And ProtoMechs in the 10+ ton range do even better then Vees in out performing Light 'Mechs and have a lower transport foot print still
I'm a huge user of Protos, and yeah, they can be really useful and really nasty. I replace certain light mechs with them all the time. But they also have enough downsides for me to not want to exclusively field them over lighter mechs.