The lifespan for draenei is kind of… up in the air, actually, as is the timeline of when they were around on Argus. Specifically, the original lore for them (original retcon lore in BC), stated that the draenei race rose on Argus 25k years ago, ergo they became a civilization at that point. The only other date we have regarding them is when Velen’s son was taken, which was supposed to be when Argus fell, which was stated to have been 13k years ago.
So going by that, the eredar could have been trucking along as a society for 12k years before Sargeras came knocking. As for their age, that in and of itself is also, now, weirdly up in the air because we now know that time passes weirdly in the Nether, which is how Turalyon is 1000 years old at this point. The naaru ships are all dimensional ships, which means they travel through the Twisting Nether to get to places. Meaning that the age of draenei might be inflated due to this, coupled with them also being in stasis during parts of the journey. The final implication of this is that draenei are like, 13k at most, and don’t actually live that long naturally. Long lived, yes, but not actually immortal.
In terms of how the orcs beat them and then the humans beat the orcs, there are actually some reasons the that, the first of which is that the orcs outnumbered the draenei massively. There were no non-combatants in the First Horde ranks, and they also had the ogres with them, demons, Fel-enslaved elementals, the Dark Star, literal biological agents in the form of the Red Mist, etc. It was basically an entire planet gone rabid against an army of defenders that maybe numbered a handful of thousands, and the last stand of the draenei still lasted for an entire day and night.
And that was the last time the Horde was ever actually at full strength. Some clans were left behind, for instance, when they crossed to Azeroth, while one clan was banished. Moreover, there were serious divisions in the Old Horde, and infighting almost immediately once they’d apparently finished off the draenei. Many of the Horde’s setbacks were basically due to their own leaders (Gul’dan) shooting themselves in the foot over and over.
So yeah, it does make some measure of sense.