The problem with the Polarize forma wasn't the number of forma (per say, for some people that might have been where they drew their line) but it was that you are being forced to polarize period.
That scenario works out very well if and only if the player either
has something they want to forma at the second the challenge drops
gives absoloutly zero heccs about tossing a forma into some item of their gear for absoloutly no reason
Since group two would gladly do it regardless, the same people who had no problem farming up a throw away AMP/Moa/Etc to complete the (already removed) Gilding challenge. So I'll focus on the group that is going to be negatively impacted by this challenge regardless of the number of forma required tho the reduction does mitigate it to some degree.
I've been on the receiving end of the "you just formad a bunch of gear two days ago, here have a Forma Challenge!" a couple of times now.
The problem that still exists is that this challenge is something that is going to interfere with peoples regular gameplay regardless of whether or not it is active by proxy of it's mere existence. Because knowing it exists players are inclined to hold off on adding a Forma to their gear naturally because of the concern that if you Forma now, the challenge could show up in the next rotation. So instead of adding the Forma when you have the need players can find themselves waiting until the challenge pops up.
This is especially true of early game, I remember a time where I would sit on Forma for upwards of a week because they where so rare. Just mulling over where I would get the most benefit from that precious resource. Now I have most of my gear where I want it anyhow and even if I didn't I have relatively easy access to forma as compared to when I started. I have always hated to waste Forma so the whole which is probably why the "Just forma something random" or "just repolarize to the same polarity" has always just sounded extra dumb to me.
At the end of the day people are going to want to complete as many challenges as are thrown at them until they hit max rank. That is just human nature. The exact same nature that saw people completing and complaining about Gilding modular items. They're going to feel bad about not doing the challenge so they are going to do it and then they are going to feel bad about having done it because they just wasted resources.
The only group this challenge really works with are people who serendipitous happen to be thinking about a specific upgrade anyhow. Which is why IMO it should be removed just as the Gilding challenge was for essentially the same reasons.
An alternative to removal that might not "solve" the problem in the long-long-longrun would be to overhaul the whole polarity system to switch over from a single polarity system to a multi-polarity system. Specifically an iterative multi-polarity system, where every forma you add (up to something like 40-50 depending on Umbra forma) would add value or at least perceived value to your weapon/frame.
There is no fear of power creep in such a system because any current min/max build has already reached peak power. The only thing you gain from an iterative multi-polarity system is build flexibility. Making such a system IMO the best possible QoL update Warframe could ever receive.
Essentially it would transition from the current system that goes
New Weapon/Frame > Min-Maxed/optimal build
New Weapon/Frame > Min-Maxed/optimal build > Build flexibility and exploration
Min-maxing will never go away but the current system either punishes not only for achieving the min-maxed meta build by locking you out of build variety but also by punishing you for daring to attempt to build differently in the event that doesn't pan out. Because a failed attempt is going to waste not just the one forma for the attempt but a second forma to get back to your working build. So for a lot of people they just never try because there is no incentive to do so.
Additionally I feel that is a significant contributing factor to a lot of underused mods not being used. Not just that people find them a bit lackluster but because it feels like you are being punished for the very attempt of trying to work them into a build.
Where as an iterative multi-polarity system if that slot was already polarized and you add a second polarity to it, now it's dual-polarity and even if your initial consideration didn't work, you can always fall back to your old build because the new polarity didn't invalidate it like the current system does. Want to try something else with that dual-polarity slot? Just toss a third Forma on it and now it's Tri-polarity, etc.
The current attempt to delve into muli-polarity with the all or nothing approach (Aura Forma) doesn't cut it as an "only option" because again you get into a place where players can feel that you are forcing them to do X when all they want is Y. Why do they want to quad-polarize a single slot when what they want is dual polarity over two slots?
However the universal polarity Forma could serve well in tandem with an iterative system because then they become a choice. They can potentially save the player leveling time and that could naturally draw players in instead of being the one UND ONLY VUN way to do it.
I think with an iterative multi-polarity system you would see a lot more people using a lot more forma and trying a lot more builds, all while feeling good about doing so. It also gives players something to grind towards for something more than "teh memes". Again without adding any additional power creep to the game because we've already hit peak power with in the current system.
Thank you for your time and putting up with my meanderings.