The early E300s (05 plate) and even the later E200s (09/59 plate) were absolutely terrible buses. Rattly, noisy, loud, with a rubbish seating layout. But they were bought in quantity because a) Stagecoach and b) price. The older E400s are pretty shonky too.
Wright made good solid buses- even the 51-plate Eclipse Geminis are still going strong- but were losing market share to those who were cheaper. Go, for instance, switched to Versas purely on price and Arriva even tried the Temsa Avenue. So the Streetlite was a partly a response to that and partly built to compete with the Solo too.
So of course Wright reduced build quality, if operators were buying Temsa bloody Avenues that told them operators wanted cheap not good. I suppose the problem is Wright always did good quality, so mediocrity was not accepted, especially by their biggest customer First (who, to their credit, never went down the cheap crappy lightweight route).
I don't really get the gushing over the E200 MMC mind, it's not exactly a luxury bus.