I'm pretty sure WOTC wanted Modern to replace Extended, but it has been plagued with the same problems of that old format: constantly broken. It has had it's moments of healthy metagames, but generally it will always cycle into an unhealthy one given enough time. Pioneer is their cold re-restart, hoping to make an Extended that doesn't need constant watching. They have been fast and furious with bannings, but that will slow down eventually I think. I can see the possibility, if small, that it isn't Legacy that dies because of Pioneer, but Modern. Modern has had an identity crisis since it's inception, something I don't think Pioneer will have.

To be slightly reductionist:

1) Vintage = all the broken shit, some of it restricted, the P9 format
2) Legacy = most of the broken shit, the Brainstorm format
3) Modern = lots of almost-broken shit, the socially awkward middle-child between Legacy and Standard, the 'go fast or go home' format
4) Standard = play the best shit available, the 'good stuff' format
5) Pioneer = play the best shit available but with a bigger card pool, the 'cool new kid' format

Who do you think gets left behind in this scenario? I don't think it's Legacy. Pioneer seems like an attempt to bridge the growing chasm between Standard and Modern, something that Modern tried to do between Legacy and Standard. However, without cards rotating out of Modern (like Extended did back in the day) it will always be an awkward bridge between Standard and Legacy. There is a fairly dedicated Legacy community, one that puts up local events all the time. I don't know if Modern has that sort of support structure if it loses steam and Pioneer ends up being the Pro-player format of choice instead. My gut tells me the Modern community is leagues away from being as strong as the Legacy community.

The craziest part is I don't think they are done yet. In another 5 years there will need to be another format, one that bridges Pioneer with Standard. At that point you can go with the Geico method, skip the middle-men and get into Legacy for a truly eternal format. The extinction of a weaker format is Legacy's gain.