If someone thinks emotionally or rationally, then it may sound like the startup company may not remain competitive if the founder/owner has exited the startup.
But at the same time, we can think that why would the startup will not be competitive any longer once it is acquired by a bigger company. If there are proper synergies in place or things go right, definitely, the startup may prosper. The startup has got the infrastructure of a bigger company and the acquirer has acquired for a reason by paying some substantial price. They would not let it die unless the acquisition process failed. I would rather think that the acquirer in this case bigger than the startup is BIG due to their process, technology, skills, etc, and has some vision. The vision for the startup by a bigger company may not completely align with the founder, but they may reach the place where they want to be in the world.