You've gotta remember that the elders really didn't know what they had and were not explicitly trying to hide them from dangerous people. Yes its true that the elders are more powerful (albiet I'd say some of the tougher illuminati are more powerful than some of the weaker elders) they really never knew what they had.
| > > |
> > ... in terms of achievements.
> > Doom will be Marvel's #1 villain for sure.
> > WOW!
> Lord Majestros:
> Thanos had to get the gems from the Elders of the Universe without Death finding out what he was up to, Doom has to get them from the Illuminati, not the same level of accomplishment. On top of that, Thanos became the 616 Universe by replacing Eternity, Doom would have to surpass that and beating a host of Celestials doesn't do it. So, just getting the gems doesn't even put Doom on par with what Thanos did, let alone surpass it. Plus, Thanos exceeded his accomplishment with the gems when he gained the Heart of the Universe which was a greater power.