While you might technically be “able” to ride in any saddle, the right saddle that fits you and doesn’t make you fight for your position is invaluable.
This is coming from someone who spent $6k on a semi-custom Antares which technically fit me and the horse, but was ultimately detrimental to my riding and position because it had too wide of a twist and was too deep seated for what I really needed. Once I switched to a relatively “off the rack” Butet, my position and mobility was immediately improved and I was able to do the things my coach was asking of me.
I sat in a multitude of different dressage saddles while shopping earlier this year, and while each “technically” fit both me and my horse, there was an enormous variance in how each felt and rode, and there was a clear winner.
So - yes, you may be able to “get away with” riding in “anything” - but it won’t be aiding your learning journey.
I really - REALLY - wish that there was a saddle company that made a truly completely adjustable saddle that could be adjusted to any horse - I would seriously spend $15k on a saddle if they could custom make it to what I need for myself and could be completely and totally adjusted to any horse I may own in the future. (Not on a daily basis, mind you.)