10k egg really?

All "eggs" are actually exact Pokemon spun randomly when you spin the stop. So no, duration of storage or hatch has zero effect on a mon that you have already spun but not revealed through hatching.

If you could get better pokemons by holding 10km longer, then everyone would have snorlaxes.

And this is not new game anymore.
This kind of game changing behaviour would have alrdy been noticed by community.

That has no impact whatsoever. If you and a friend pick up the same egg it will lead to the same Pokémon (same IVs and moves). Therefore, since you both have the same Pokémon, timing on the actual hatch doesn't matter.

Hope this helps and makes sense.

Egg hatches are 100% without a doubt determined when you get them. Whether they are influenced by location is debatable. There have been thousands of tests on this but the evidence includes: A.) If you hold onto an egg from level 10 and hatch it at 30, the level of the mon’ will correspond with a level 10 trainer. B.) If you got an egg before gen 2 release, you 100% cannot get a gen 2 mon in it.

The mon you get is determined from the exact moment you get it from stop, and it also has been proofed that even the distance of the egg (2k, 5k or 10k) means nothing about the mon you will get, each mon has it's own % of hatching, so when you spin the stop you will have for example a 0.1% of it being a "insert mon name".

I myself just hatched 5 10k eggs I had only for 3 days, and 3/5 where Larvitar, and they were since the moment I picked them up during this past weekend.

Where do people get these ideas that the longer you hold an egg the better it will be?

The pokemon you get from the egg is determined when you spin the pokestop.

Everything in the egg (Pokemon spieces, moveset, IVs) is determined when you pick it up so i doesn't matter if you keep it longer or not

If I have a couple of 10k eggs I sit on my phone and keep it warm. I sing to it and I swear they come out Snorlaxes and Larvitars. You must infuse your eggs with care and affection. Otherwise you get Gilgars. It's like getting coal in your xmas stocking.