Previously I combined directly to her Sapphire Preferred, but in the transition from Preferred to Reserve I changed to Freedom because I knew that card would remain open long term.
That is exactly what I do as well for the exact same reason.
Although I often just let the points at my Freedom sit. May be I need to be more diligent and do it monthly.
In the past few years there were actually gap of months when we did not have any card had transferability. Of course it depends on each person's situation but I assume many really dont need to maintain a fee-based card to keep the transferability all the time.