Iirc, INNO has stated several times that spending diamonds is not required in order to acquire a fully levelled event building. Afaik, they never said how easy, or how difficult, getting the upgraded building would be. Based on the reports of many players, no diamonds were necessary to get the Pizzeria/Bistro this year. Some players were not successful, but this happens every event. Did getting the building become more difficult? It didn't for me but YMMV.
It is objectively more difficult this year than last comparing same event to event. About 1000 more footballs for 1 more upgrade needed. 1000 more footballs doesn't get you 160 yards under any strategy, so some of the extra required yardage has to come from better choices, better luck, or the cushion you cleared the event by anyways last year.
The cost of the diamond coach increasing can also be viewed as an increase in difficulty for those whose strategy hinged on paying for it with free diamonds (this time they have to have made more free diamonds).
----
Is it *too difficult* is a subjective matter. I didn't find so. Some did.
It is partially obscured by luck - i.e. two players following identical "+1 yd coach, only shortest" strategies, one with good incident luck and one with awful incident luck have different results : good incident luck = i finished it easy, what's your problem ; awful incident luck = "it was impossible" (it wasn't but it required a different strategy to compensate for the bad luck)