I guess people mostly downvote you because your statement is not popular, but it's not quite correct either. 'Subsistence farming' isn't one well defined thing. There are form of subsistence farming that are very bad; for example, I'm doing work on agriculture in Madagascar now where people (due to a lack of knowledge and systemic thinking) do agriculture by slashing down rainforest to get access to the very fertile soil underneath. That soil inevitably washes out fast (because the trees and more in particular the roots that hold the soil are removed), so a few seasons later they move on to the next patch of rain forest and so on. This is one of the major environmental threats on the island.
That said, 'subsistence farming' in temperate climates as it was practiced say in the 18th and 19th century in Europe is quite different. It revolved around permanent settlements (so people couldn't move on quite as easily) and used crop rotation and animal manure for keeping productivity if not high then at least at sufficient levels. We couldn't live from a system like that today, and it was bad for the environment in the sense that it made people put essentially all land into cultivation, but it wasn't as immediately destructive as other forms of subsistence farming are.
That said, 'subsistence farming' in temperate climates as it was practiced say in the 18th and 19th century in Europe is quite different. It revolved around permanent settlements (so people couldn't move on quite as easily) and used crop rotation and animal manure for keeping productivity if not high then at least at sufficient levels. We couldn't live from a system like that today, and it was bad for the environment in the sense that it made people put essentially all land into cultivation, but it wasn't as immediately destructive as other forms of subsistence farming are.