"Given MVF and VRT's fertility specialists are primarily paid...

"Given MVF and VRT's fertility specialists are primarily paid based on volume", I'm not 100% sure how true that actually is. Certainly from a small sample the two main payment structures for IVF specialists at one of MVF's clinics is via
1.salary with bonus (I'm not sure of the bonus make-up but could well be volume related, but the bonus is a smallish portion of their take home cf financial industry),
2. and the majority are paid via a percentage of Billings which is individually negotiated.

The doctors that are very popular via word of mouth or social media have the option to leave (usually with non-competes fort least 12 months) and then may choose to use mvf or vrt for the technical aspects and the diagnostics anyway but MVF and VRT would certainly earn less under these conditions. This is not common however and takes a very driven, financially sharp and charismatic doctor, which most are not.

Setting up an IVF lab is certainly a very large barrier to entry but as you said, with high ebitda margins, it can be achieved by corporates like PRY. There are smaller partnership groups which can be very profitable and can set up the labs, but this is tough risky business for doctors who can otherwise 'relax' under the umbrella of a corporate.

I certainly don't know which way the margins will head in the next year or so, but the fcf yields to current EV do account for further profit weakness IMO.

My two main points are:
I believe particularly due to the increasing number (and particularly female heavy) of fertility specialists, that MVF and VRT will always be able to fill the seats required, and the more doctors there are, the better the corporates bargaining power. The other thing to note is that female specialists tend to be more focused on family and job satisfaction, rather than maximising profit. (I hope no one finds that offensive, it certainly isn't meant to be, and if anything is an indictment on males!)

Secondly, because referrals for fertility services often are driven by the patient, rather than the gp, advertising and marketing are very important in this space, and if VRT and MVF can 'sell' their best in class products, they will surely maintain demand. The scale of these companies makes advertising much more efficient than for small practices. This is the space that VRT and MVF need to excel in.

In terms of supply, the fertility specialists are highly paid generally (read, high margins), and so comparison to GPs and dentists may account for the difference in the EBITDA margins cf ONT and ZNT (obviously the gp aspect of ZNT is relatively small and diminishing - which to me is a good thing, as ZNT's allied health and aged care focus is a gold mine imv).

I would be interesting in your, and @madamswer, modelling to see how far margins could fall and still give a decent 15% pa return in the medium term (4-5 years).