With proper care and maintenance, they should last well over a hundred years. 50 years is perfectly fine.
Apples and oranges. You can't compare a mature city in a first world country like Tokyo to Beijing. Tokyo has a lot more in common with London and the Olympics worked out great for them.
A major part of the current administration's strategy for economic revival is to significantly increase the number of foreign tourists and the Olympics are one of the tools that will likely make that happen. The whole point is to get people to come over and continue to do so. People getting screwed over logo design might suck from a moral standpoint but it doesn't matter to the economy.
東京オリンピック 競技場 会場 最新情報 東京ベイゾーン ヘリテッジゾーン 見直し 整備計画 仮設施設 - "メディア・クローズアップ"放送・通信・ICT・メディア最新情報-国際メディアサービスシステム研究所
"競技開催計画では、既存施設15か所、新設施設22か所(恒久施設11 仮設施設11)"
15 out of 37 are preexisting, 22 are new but half of them are only temporary. So basically they're building 4 fewer (permanent) new venues than at the last Olympics.