From 1970 to 1990 there was collectively not more than about 30 types of 'O' Scale engines coming to the market from Lionel, Williams, MDK, Weaver or MTH. Part of that reason is that Weaver only had put out one Scale engine by 1990, MDK had put out one and MTH had yet to enter that market.
Between 1990 an 2000 Lionel was doing very little in the Scale area until the late 90's excluding the MTH production made for them earlier. Williams was putting out a steady stream of scale products. Weaver thru the association with MTH thru around 1996 had put out several scale engines. MDK had yet to focus their attention on Scale and was carving out market share in 027 sets and some scale rolling stock. They probably should have kept that focus. At least they made trains that had a great deal of play value. MTH after the 1993 issue of the Dash 8 was just beginning to focus what was to become the largest ever seen production of O Scale engines in my life time (age 77) and probably ever.
From 1995 thru say 2007 when Williams sells and MDK ceases independent operation there were a lot of players in the market producing what turned out to be an over production of 'O' Scale engines. Weaver is out by 2015. And, as we know MTH is out by 2021.
What does this mean for the future? Don't plan your retirement based on selling all your really nice Scale engines for anything approaching your original purchase price. There are too many engines and we are dying too fast a clip. If as most people seem to believe, that operation of trains in the future will be based on the electronic wonders to behold, then the older Lionel will have a downward spiral because all it does is go around in a circle, doesn't talk to you, doesn't talk to itself and starts too fast.
My approach hasn't changed. I like my trains, I will keep buying them and I really don't care what my kids do with them.