You don't know what a child's needs are without assessment. That's why you get it done. Otherwise, you could just implement provision (or not, as the case might be) at will.
Early intervention is key - everyone agrees on that. But if you only assess because there are clearly awful problems, by definition, you have identified the problem late. Far better to go upstream and work out what a child needs before they are evincing damage.
We didn't get my son fully assessed until he was ten. We had my daughter assessed at 6/7. Guess which child had a high needs EHCP at an age early enough to avoid developing a range of diagnosed mental health problems, on top of the autism diagnosis (we had that assessment done when he was five, but by itself, it's just a piece of paper).
My gifted daughter had issues only identified by thorough cognitive assessment. She had 99.95 centile results for reading and maths at the age of 7, in stark contrast to her 52nd centile processing speed. My son's scores were similar, but by the time his were done, That's actually fairly normal with autism, but at the time, she wasn't diagnosed and the school were insisting there were no needs at all, because she was so bright. This was a pattern repeat of what had happened with my son. Both also have major problems around executive function (my daughter's EHCP mandates the Smart But Scattered programme) which again, in a very able child can be effectively masked for years, harming their learning badly. My daughter is thriving in a small independent primary with a lot of extra support - which she only has because we had the full assessments done early. My son has a high needs EOTAS - he costs the LA mid-5-figures, despite being educated entirely at home because there is literally no school they can name able to meet his very complex needs. That is from the harm done, by his needs going unrecognised and unmet for so long.
Sometimes, it's the children who are seemingly very able and with no L&C needs who most need the assessment, because their issues are completely invisible until they cause significant harm - academic and emotional alike.
My son has PTSD, which isn't unusual in able autistic children completely failed by the mainstream school system. If he'd had proper assessment at the same age my daughter did - which includes full cognitive testing - he could have been spared a lot of suffering, as well as a lot of academic struggling when he reached the age where being very bright is insufficient, by itself.
What is unethical is ignoring potential problems until you have caused harm to a child by that ignorance.