I'm honestly having a hard time parsing your argument in a way that isn't "Democratic presidents are automatically corrupted by Wall Street just by virtue of being president."
Let's start from the start.
1. At least some Democratic politicians might be inclined to change the way they vote on a particular bill if there was some reward at the end of it. We can identify two mechanisms for this:
a. conscious corruption. They change a line in a bill, in return, they get $2,000 in speaking fees after leaving office.
b. soft influence. They get to pal around with wealthy people, get invited to all the parties and soirees, get told they'll have a great speaking gig after they leave office. They don't consciously mean to change how they operate, but subconsciously they're now predisposed to be more favourable to moneyed interests.
2. Accordingly, it would be better if we discouraged Democratic politicians receiving inducements from moneyed interests, including a good grace period after leaving office, to prevent the way Democratic politicians vote or author bills being influenced.
3. This is true even if there are some (most) Democratic politicians who are uninfluenced and genuinely morally upright. I.e., I'm reasonably confident that the fee Obama received did not influence him, from what I know of his character. However, I can't say that in confidence about all Democratic politicians, and so I would rather have this safeguard mentioned in 2. in place.
4. Obama is a senior Democratic politician with significant influence over the party. Future Democrats will seek to emulate him, his actions form a model for what they do or do not do. Given 2., it would therefore have been better if Obama had chosen not to do this, since Obama refusing to do so would help prevent others, who may be influenced under 1., from doing so.
Saying PBY's argument is "Democratic presidents are automatically corrupted by Wall Street" is a straw man. It looks nothing like the argument set out above, which I think is a fairly pedantically constructed version of what PBY believes.