I had anticipated that RJ would be much more serviceable as a make-shift primary ball handler by now. Tatum and Ingram (and heck, Zion) developed into better ball distributors than RJ more quickly.
Luke Walton made Ingram play PG for stretches on the Lakers and you can say that helped set the stage for his development years later. This season he averaged 5.8apg.
As to why would NO want to deal Ingram? For one, he has a max contract and if he can keep himself healthy enough to get on an all-NBA team, he can become a very costly supermax contract on a physically fragile player.
The team's big issue is the health status of Zion. Maybe it makes sense for them to move from Zion; alternatively, you can move Ingram for a package of RJ & Obi and/or assets. While RJ has been a less dynamic player compared to Ingram, he costs dramatically less. Plus with Zion's propensity for long term injuries, you ideally want a high upside backup/part time starter in that role. They were manning the position with 6'7" Herbert Jones. So the wins are: more depth, financial relief, all with upside.
The Knicks would take on the financial burden of a potential supermax deal and a less durable player for the benefit of all-around better play.