That is exactly my point!
For more complex animals such as cats and dogs, it seems to be easily observable. They remember things, they are attached to ‘their’ toys, places, people, they have moods and seem to definitely experience dukkha.
But it doesn’t seem as easy to observe in lower animals. The further down the food chain one goes the more difficult it becomes.
And we meet an added complexity - the difficulty of ascertaining if what seems to be vinnana is actually that and not an artefact. For example can an ant differentiate its nest from another or is it just a chemotactic response?
And then there are the endless possibilities of AI… could a self driving car which seems to remember, plan and respond develop a sense of Self? Even a sense of Self it seems, can be programmed.
So I guess, I sympathize with our early buddhists who decided to give the benefit of the doubt to the apparently less developed creatures, out of compassion. BUT, I feel that going to the other extreme like the Jains do would be a bit over the top too. 