MTH and others flooded the market with new product. This drove down prices of older Lionel - especially. MPC - over time. But it also made entry into the hobby more affordable.
MTH also pushed Lionel to innovate with new features, especially electronics. However, this came at a cost as prices increased on newer stuff.
MTH forced the move of production to China. That was a two-edged sword as we all now realize.
All in all, MTH has been a very positive influence, but not without some fallout along the way. Consumers certainly have a lot more product choices than ever before; whether that is sustainable over the long haul is a real question.