There is no precedent for a fields medal just building foundational theory. Lurie is not comparable to Grothendieck at all, the way they worked was completely different. Grothendieck proved GRR by age 28 (!), Lurie still hasn’t proven a big result of that impact; the stuff about elliptic cohomology and cobordism hypothesis has not been taken up outside algebraic topology and higher categories theory.
Jacob Lurie proved the Baez-Dolan Cobordism hypothesis. Along with Denis Gaitsgory, he proved Seigel-Mass formula for function fields. Saying that Jacob Lurie didn’t prove any “big” theorem is like saying Grothendeick never proved any “big” theorem.
Also his geometric construction of tmf
I already mentioned the cobordism hypothesis and tmf (that comes under elliptic cohomology), and like I said, no one really cares outside homotopy theory and related areas. Ok, there is some application in mathematical physics, but his controbution in MP can’t be compared with other FMs like Okounkov, Kontsevich, Donaldson, Witten, etc
Tamagawa number stuff with Gaitsgory was too late for fields I’m pretty sure, and anyway it’s not a conjecture anyone really posed beforehand. Has it been applied for anything? Don’t think so.
Grothendieck had GRR at age 28. I’m not even going into everything he did later. Where is Lurie’s GRR? You may not realize the impact GRR has had in AG and related areas (even that has been applied more in math physics, through stringy stuff, than cobordism hypothesis actually).