Wat Phra Dhammakaya is alleged to have used money to garner support of senior monks throughout Thailand to try and monopolise Buddhism in Thailand. This is clearly a conspiracy theory that is based on no solid facts, nor is it a plausible theory for many reasons.
Thailand has many senior monks who have been ordained for 50-70 years and have devoted much of their lives to Buddhism. It would go against all of their doctrines and moral codes as devoted monks to simply be bought by money to destroy Buddhism. Also, there are over 40,000 temples throughout the whole country. Even if by any chance it was easy to “buy” senior monks, there are too many temples. No one would be able to take over that many temples in just one generation, so there is no reason to actually even try to do so.