Sure, this isn't the most exciting change, but it's necessary.
Full decently rolled ancient gear can be obtained after perhaps 100-200 hours into a season if you work straight towards that goal. At that point, with full level 80ish augment you can expect to have about 16k mainstat points. An additional 200 paragon levels gives another 1k mainstat points yielding a 6.25% increase in damage (and some associated survivability). The rate of return on this does diminish, but pitifully slowly. At 25k main stat points (around paragon 2600 or so) an additional 200 paragon levels is still giving an additional 4% damage increase.
By the time you have full augmented ancient, its going to take considerably longer to squeak 6.25% more damage from gear upgrades than it would to grind 200 paragon levels. If you ever get 2k+ paragon your gear would probably be nearly capped out. It's unlikely to even be possible to get 4% more damage out of gear upgrades by that point.
The end game progression turns into nothing more than a horrible slog of grinding paragon. There isn't isn't even another option on how to progress a given character let alone a competitive or compelling one.
Introducing super rare, more powerful versions of existing items greatly extends the longevity of the item hunt. I'm not sure it would extend all the way up to 2k+ paragon, but it would easily extend to 1500ish if not higher. Super rare meaningful items are necessary if the item hunt is ever to compete with the paragon hunt.
And besides, implementing primal ancients isn't exactly a huge time sink for the dev team. This is an easy and necessary improvement. It will shift the mid and late game back towards item hunting and further delay the importance of paragon grinding until levels past what most people ever achieve in a season.
I definitely want to see more new items, better balancing of builds, and other horizontal progression options. But for vertical progression, the benefits of implementing primal ancients FAR outweighs the costs. It's a no brainer.