It's the same as "you must spray off the old sward ". If the new sward is as good as the salesmen say it will outcompete the old sward. (which would have benefited greatly from 2 ton of lime and 2 cwt of fert an acre)Chicken and egg? The more you spend preparing seedbeds, the less gripe you'll have spending big dough on little bags of seed, because as a fraction it is "less expensive"
likewise, after you spend hundreds on preparations, break crops so the "pests" that normally control your weed seeds don't eat the grass seeds.... you need good results. And it all hinges on timely rainfall
One great thing about tough times (whether economic, climatic, or other) is that they really show these shiny-shoed 'experts' off for what they really are - people who work in sales, like the rest of us