Yes some of the teachin has been decay, but not the main teachings. this is why it still are valid today. and it work for me
From my investigation and research I believe the main core teachings of Buddha have been lost and changed by men to try and say He didn’t teach about God. But when I read things like this from Buddha I tend to think that the Buddhism being taught today is not what Gautama taught.
There is, O monks, an unborn, unoriginated, uncreated, unformed. Were there not, O monks, this unborn, unoriginated, uncreated, unformed, there would be no escape from the world of the born, originated, created, formed. Since, O monks, there is an unborn, unoriginated, uncreated and unformed, therefore is there an escape from the born, originated, created, formed."
The Three Characteristics And The Uncreated