Unfortunately, DC whose parents know the LA's statutory duties, can navigate the system and fight for provision get the best support.
Legally there is a distinction between Compulsory School Age, which finishes at the end of Y11, and the Participation Age, 16-18. Education isn't compulsory for 16-18y/o's, education, training or employment (that also has an element of education or training) is. Although no one follows up those that are NEET.
DC that are CSA are entitled to medical needs tuition which, to an extent, assesses their needs and is tailored to them. However, it is not comparable to a needs assessment or finalised EHCP.
An EHCNA is more than just an assessment by "an education expert". It gives the right to request assessment from anyone reasonable. An EHCP is legally binding, whereas anything less isn't.
I wouldn't want to give up the right to an EHCNA for an automatic assessment from "an education expert" because, no doubt, it would be less comprehensive and the outcomes unenforceable.
Also, not everyone having medical needs tuition will need a full assessment. A child out of education because of their SEN meets the legal threshold (has or may have SEN and may need SEN provision via an EHCP.) for an ECHNA.
Money spent on assessments is a drop in the ocean compared to the money LAs spend defending indefensible cases at tribunals. If only the general public knew how much was wasted.