!!! YE BE WARNED - SPOILERS AHEAD !!!
Felt the same but then after thinking about it for a while I came to conclussion that it was the best way to end this game, Shep had to commit ultimate sacrifice to save galaxy, Reaper tech had to be destroyed so the cycle is broken once and for all (remember what Sovereign told to Shepard on Vermire, that every major space faring race evolves in the way Reapers want because of their tech spread through the galaxy ? (namely mass relays and citadel) so in order to stop it those had to be destroyed.
I also thought it was uber sad for Normandy crew, but after credits we also get that cinematic clearly showing the same planet Normandy crashed on several centuries later so the crew, although very unlikely, did survive and managed to establish colony which grew to quite a population judging by the dialog (they had to achieve certain level of advancement to speak about traveling space soon.
So all in all, Krogans are free from their disease and now can rebuilt themselves independantly, Turians and Asari also will have to live on their own, Earth is saved and also separated from everyone else, but they keep the tech and eventually will rise from ashes like other races. Depending on how many Quarians went to the battle and how many stood on their reclaimed world they either extinct or somehow survive and continue living with Geth in peace (provided that you managed to bring them to peace), Joker and Co. establishes colony and lives happily till the end of his happy life.
THAT IS OPTIMISTIC VARIANT
NOW IT'S TIME FOR REALISTIC
Shepard dies for nothing, Reaper tech destroyed, severs all major races from eachother, colonies have to live on their own (without supplies, they eventually die) Turians looses their homeworld due to Krogan expansion (genofage cured, madness unleashed) Asari have to struggle with all that pureblood crap (at least they don't have krogans around), Quarians left on their planet fails to adjust to 'new' ecosystem and eventually die, Geth coninue to expand (provided you didn't choose 'destruction of synthetics ending'). Earth, or what's left of it is in total chaos, Turians starves to death, Krogans starts to overrun the planet killing everything in their way, Citadel wreckedge falls down to the Earth surface killing countless of lives, whole allied fleet is stranded in SOL and might aswell share the fate of Turians (I remember Turians can't digest human food and so does the Krogan and Salarian, Quarinas would eventually die too due to their immune problems.
Normandy crew stranded on the tropical 'paradise' full of carnivoreus critters of all sorts with only handful of military personel, food and weapons... either they die due to incompatibility with environment (see the 'paradise' planet from ME2 where Jacob's dad crashed) or they will die naturally due to too small gene pool ( no way they could establish healthy population that would grow for centuries eventually hitting the same level of advancement as their ancestors had - namely Normandy crew - there simply to few of them + some of them were alines, robots or cripples ...)
It really depends on your imagination, at first I was realistic about it but then I went for optimistic point of view
