I'm also experiencing this issue.
I walked away from Warframe after being a veteran of roughly 6-7 years because I burnt out trying to regrind everything I had on Xbox. When I transitioned to PC, after playing on Xbox for about 4-5 years, they had previously made statements indicating that account merges would never be possible due to platform restrictions. This is why I felt comfortable transferring my newly made PC account to the Switch, just in case I wanted to play the game while on vacation/business trips, though that never happened.
I only launched the game on Switch one time, but I still did it with full confidence that I was supporting DE in their endeavor to reach new platforms. It felt like I was helping them reach new audiences, even if I didn't actually use the cross-platform feature at all. I even remember thinking, "Maybe this will help them figure out how to do merges at some point..." LOL
Low and behold, they're now possible, but because I wanted to check out their new shiny cross-platform save feature on the Switch YEARS ago, I must either moan and complain until an acknowledgment is made, or choose to completely erase my history with the game on Xbox. I've been slapped in the face by the same company I thought I was supporting.
Logically, I understand that some things are more complicated to accomplish than they're worth, but my recently renewed interest in the game has been completely soured by this false hope they've given out. If my time invested is not worth the technical effort it would take to administer methods that can successfully merge these accounts, then I can't imagine they value any of the time I have, or would have, put into the game on PC.
They've created a half-baked solution to a fundamental problem in their ecosystem. They seem to have just assumed it would go over well, or that the vocal minority of us that suffer as a result will be too few to have any impact on their day-to-day operations. Unfortunately, this is very likely to be true, so I can't imagine we will ever see solutions for this that benefit all players. It's always been like pulling teeth with this company to include features/adjustments that the community wants/would benefit from. After all, for every one of us that leaves the game because of how they've neglected those that previously migrated, there will be two more that spend time, money, and effort regrinding what is still available. As a business, that's a loss they're going to be more than willing to take.
At the very least, I'm glad this happened before I got sucked back into playing Warframe again. Now, it's so much easier for me to go be a drop in the bucket for some other company that does a better job of maintaining the illusion of player respect.