Generally speaking, the spine is never straight. A straight spine will not bear body weight in a bipedal or standing position.
Further, a neutral spine, one where the three inherent curves are preserved, is not an “automatic” from Padmasana. It is quite possible to flex/extend the lumbar, thoracic, and cervical spine while in Padmasana.
And finally, even if a neutral spine were an automatic by-product of padmasana, a neutral spine for meditation at the expense of knee damage does not seem to be all that fruitful a meditation and certainly would be up for debate as to whether it was “peaceful”.
Meditation can be achieved by a practitioner in several positions. That is the overarching principle and why “others” say Padmasana is not necessary for meditation to occur. Just because you or I ache at the 45 minute mark when meditating in Virasana or Vajrasana or Sukhasana does not necessarily mean Padmasana is the only answer.
Everyone has to “do some effort” to maintain their sitting.