I'm confused... the OP mentions that the exponential growth rate of paragon experience needed caps out at ~170 billion. Yet this is farmable via group play speed farming in sub-difficult GRs in an hour (or so). First, let me propose this: The group or groups of players who choose to farm/leech experience, usually band together for 8-12 hour blocks and constantly farm. Should this play style be rewarded? Absolutely. You argue time spent vs reward, the 4 hr a day player will always be left in the dust due to the generic loot system that D3 holds regardless of paragon. Now, to your original point, paragon ep required from 1999 to 2000 and 2000+ is the same? Good. 170 billion per paragon seems grim, but solo farming non-stop T6 rifts you cap out at approx 2.5-3 billion exp/hour. The "faster" you speak of doesn't exist, if you've achieved paragon 2000, you've most likely gotten perfect rolls in every slot. The generic power creep of paragon does exist, but if you take that away, all blizzard will be left with is the casual mediocre players that are crying about not being able to hit the leaderboards. My suggestion is as follows:
Initiate a plan at season start: Form a group of 4 friends that play at roughly the same times/lengths you do, and decide who is filling what role for the majority of the beginning of the season, and play with them, even at 4 hours a day, you'll still be way out in front of the duos and the solos... Would you really enjoy it if they complained that you were ahead of them because you have group play?
The entire dynamic of threads like these are actually relieving, that even for solo players, there is a point where paragons become a steady earning, instead of an impossible mountain to climb.
As for botting... everybody screams bot, but without proof... Yea sure maybe they did... Maybe they didn't... Maybe they just had a better plan than you and achieved goals much more quickly than you did, and that head start has kept them ahead.