We have less than 600 pilots now, but we should be adding in the region of 200 new pilots annually for the foreseeable future (based on crewing requirements for 1 aircraft delivery per month).
So it's really simple: you join as an FO today and in 3 years time you are practically guaranteed to be in the top half of seniority without even accounting for your seat seniority due to upgrades ahead of you. The longer you wait to join though, the more time it will take to improve your seniority.
As for who gets to enjoy their roster, I would say that depends on your definition. If your definition of enjoying your roster is to pick and choose each and every flight and day off on the exact days you want them, then that is reserved to the guys in the top 5 percent, and it will quite honestly never happen for you as a new joiner going forward. It's also a matter of attitude. I have flown with senior guys who wouldn't stop whining about getting 1 "assigned" flight that they hadn't requested out of 10!!
But if you can enjoy getting one of your preferences (maybe no. 5 or 6) almost all the time, while avoiding war zones and deep nights, and getting more than half of your requested time off exactly when you want it, then being in the top ⅔ of seniority is sufficient.