Yes, after the Eighth Bodhisattva Bhumi it is already
irreversible, and Eighth Bhumi is also known as the
non-retrogression stage, or 'Bu Dong Di'. And from the 10th Bhumi
it is known as "The Path of Non-Meditation", which means no more
training is necessary.
But why should Buddha meditate? I think firstly it is to set a good
example for other Buddhists to meditate, it shows a balance between
mundane life (of preaching the dharma, etc) and absorption in
Nirvanic bliss, secondly meditation has a re-vitalising effect and
the Buddha is thus able to rest his body, in fact I read somewhere
that a short while of meditation for an advanced meditator can
equal to hours of sleep. The Buddha is known to sleep only 1 hour a
day. I think he meditates more than he sleeps.