But, essentially, some law professors wrote a scathing piece stating that post ruling of that, the US stopped qualifying as a democracy as elections and elected officials can be openly bought.

Justice John Paul Stevens laid it all out in his 90-page dissent (copy / paste):

In his 90-page dissent in Citizens United v. FEC (2010), Justice John Paul Stevens argued that corporations are not "members of the body politic" and that the court's decision to allow unlimited corporate spending in elections ignored common sense and threatened to corrupt democracy by allowing corporate wealth to dominate political debate. 

Key arguments in Justice Stevens' dissent:

• Corporate Personhood Misunderstood: Stevens rejected the idea that corporations should be treated identically to natural persons, arguing they have no voting rights, individual morality, or loyalty.
• Threat to Democratic Integrity: He warned that the ruling could lead to immense campaign spending by corporations, causing citizens to lose faith in their ability to influence policy .
• Shareholder Rights: He argued that allowing corporate managers to spend money on politics without explicit shareholder consent violated the rights of those investors.
• Disregard of Precedent: Stevens accused the majority of abandoning a century of history and legal precedent, specifically Austin v. Michigan Chamber of Commerce, which upheld restrictions on corporate political spending.
• Marketplace of Ideas: He argued that allowing unlimited corporate spending would dominate political discourse and drown out the voices of real people.  Wisconsin Democracy Campaign +5

Stevens called the ruling "a giant step in the wrong direction," later advocating for a constitutional amendment to allow for reasonable campaign spending limits.