I'm just some guy, so I don't have any insights into how they're planning or budgeting for anything.
Nessie is a beloved legacy ride from a long-defunct manufacturer (yes, S&S owns the IP and whatever leftover assets from Arrow but are otherwise not factored into the conversation), and its loops are a literal icon of the park.
Alpie is an amazing and forceful ride with good theming integration, but I'd argue nowhere near as beloved and iconic as Nessie is.
I would love for it to stick around forever. But I also know in business terms it's a depreciating asset. And unlike Nessie, I believe it's got far more complex (expensive) infrastructure that keeps it running.
There's also some known areas for improvement a simple retracking may not be able to accomplish - notably, removing the need for the elevator floor in the station and toning down forces in some areas that are overstressing the track and/or supports.
Based on what I understand about grandfather rules for the RPA buffer, JCC permitting approvals (including for the track being really close to Kingsmill) and the relative difficulty and expense of fully removing the ride that would add a lot of extra time and expense to completely replace the ride with a different attraction in that space are maybe the most compelling reasons why a retracking would be possible.