>How can we ensure say, that Microsoft doesn't pay someone to throw a wrench in libre office development or Adobe to sabotage Gimp?

Microsoft and Adobe have reputations to uphold long into the future.

Is that infallible? Hell no it isn't, but consider that Jia Tan only needed to uphold his reputation insofar as getting his backdoor onto everyone's systems. Once that is done, his reputation or the lack thereof becomes a non-issue because his work is done. We're lucky we caught him just before the finish line.

The likelihood of demonstrably reputable parties like Microsoft and Adobe poisoning the well is practically nil because they don't have a finish line.