This reads like "How about instead of the current game, we had a different game."
Lots of the time invested in designing new PVE content would be spent on making PVP work properly instead, which means that both will take longer to complete.
While it would be interesting to see a developer make something that was PVP focused with a Warframe aesthetic, Warframe is designed and balanced for PVE. It would be heaps of work to re-balance the weapons, abilities, and the basic gameplay which is all designed around fighting computer controlled enemies with your team of human controlled friends. It's just not made for people fighting against other people.
So many of the mechanics like damage resistance, armour, shield and health regen, evasion, blocking, rivens, etc fit around powering up to beat the different enemy factions and boost the types of equipment you might use against them. There are so many frames and weapons based around big aoe attacks, specific damage types, enemy mind control, damage over time, and holding or stunning enemies in place. Some of the Warframe abilities are just total invisibility or immunity to all damage which is really boring to fight. All of these mechanics are incredibly frustrating to deal with when you are fighting against a person, to the point that people avoid certain boss fights because it's just so tedious to go through those invincibility phases.