Collecting the necessary data is not a problem with single seaters. The 7+1 missions were basically check flights to confirm what has been learnt form docs and in the simulator.
From https://www.vbs.admin.ch/content/vbs-internet/de/die-schweizer-armee/sicherheit-im-luftraum.detail.nsb.html/75494.html run through google translator:

The flight testing included eight missions for each candidate, conducted with one or two fighter jets. Seven of the eight missions included given tasks. A mission could be chosen by the candidate to repeat one of the given missions or to demonstrate special features.

The aim was to check the sensors in the Swiss environment, the compatibility with the technical infrastructure of Switzerland and the degree of maturity of the fighter aircraft. In addition, noise measurements were carried out by the EMPA in Payerne and in Meiringen.

There are issues with not having twin seaters though.
With the Swiss AF not operating advanced jet trainers, going from a PC-21 turboprop directly to an F-35 seems like a big jump.
And there’s the usual. No incentive flights, not only for VIPs etc., but also for staff like flight doctors. Or desk jockeys wanting to keep their licence. LM should develop an F-35D imho.