Scarf Jirachi is not at all a good replacement for Scarf Genesect. Scarf Jirachi is an average at best revenge killer and is nowhere comparable to Genesect. It's far weaker, has less powerful coverage, and doesn't have STAB on its U-turn. Part of what made Scarf Genesect so deadly was its strong elemental coverage, and Genesect could punish efforts to avoid that elemental coverage with its powerful U-turn. Jirachi does nothing of the sort. Its coverage moves are far more punishing to be locked into, and its U-turn does pitiful damage. The lack of STAB on U-turn forces it to forgo either a coverage move, its STAB Iron Head, Trick, or U-turn itself. You can't have all of them, and without all of them Scarf Jirachi can't even come close to Scarf Genesect's level. It isn't nearly as efficient as Genesect's usual movepool was. And Genesect at least had Explosion if you were desperate for a way to stop Modest Volcarona. If you're using Scarf Jirachi it's probably because you needed both a Scarfer and a Dragon-type or ExtremeSpeed resistance. Otherwise it is an incredibly underwhelming set.
I will add to parts of this I agree with in that ScarfJirachi was capable of outspeeding any unboosted (+2 Spd from Rock Polish) Genesect and OHKO with Fire Punch. This was one of the reasons I often suggested Jirachi and also due to Trick locking the opponents' Ground/Fire-type moves so you could set up either Landorus or Terrakion respectively, depending on the the opponent's choice-locked move.
Not only that, but Genesect was praised for being a "Dragon Slayer" yet failed to outspeed one of the most common: MoxieMence. At least Jirachi was able to speed tie with Salamence in tight situations, whilst ensuring ScarfGenesect leads were knocked out on the first turn.
I'm not saying Jirachi with Choice Scarf is no longer a viable option because Trick and flinching w/ elemental coverage is always a safe option to fall back on, but it
was more useful during the Genesect era for sure.
Once more, now that Genesect is banned I can safely say Life Orb Scizor running a Jolly nature is extremely good again. You still have to deal with Tentacruel and whittle Keldeo down towards KO range, but it can sweep rain teams and sand teams with minimal effort. Really, Breloom and Scizor should begin to see more usage together if I were believe what I am saying is true.