There are plenty of other factors that influence investment rates beyond profits, including red tape, legal regulations, and market conditions. A far more likely outcome would be that board members and shareholders get richer, and the quality of the product stays the same. Additionally, some studies indicate that investment incentives would not diminish in the net neutrality paradigm as the ISP's claim.
Even the companies themselves have said that net neutrality laws do not impact infrastructure investments. In 2005, Verizon issued a statement categorically stating that infrastructure investment was not correlated with net neutrality decisions.
Instead of falling, infrastructure spending has actually increased in the US since net neutrality laws came into effect. 2016 saw a large increase in infrastructure spending from the nation’s biggest internet service provider.