Most POW mining is powered by renewables, and the fraction that isn't is continuously motivated to power-optimize by innovating in either computing or power efficiency.
Any miners paying less or the same as the lowest have no motivation at all. And dirty energy (e.g. unscrubbed coal) or energy from backroom dealings (tons of mining arrangements in eastern Europe and China) will always outcompete any legitimate competitors using renewable.
That's like saying "Apple is already the biggest company in the world, they have no motivation at all to keep making new, good products." Obviously they are under continuous threat of not being the marginal best.