The TFA uses a false model for computing profits. In the USA nearly all electric utilities are regulated monopolies. The government grants them a monopoply for a particular service area. The utility fronts the capital investment (historically up to 20% of all capital investment in the whole country!!! They must raise the capital in the private markets and convince investors to invest in utilites instead of Apple or Alibaba. High returns are needed to attract that money.). The pubic service commission is obligated to allow rates that guarantee the utility a defined return on investment profit. In real life, there is a lot of wiggle room and lots of politics in rate setting, but competitive pressure is not a factor. TFA ignores this.
We could, as a matter of public policy, decide to revoke the monopoly. That would open the door to any competitor, but it would also allow the utility to charge any rate they like without asking permission, and would remove any obligations regarding reliability and quality of service. (Think daily brownouts for anyone who doesn't pay for "premium service" on the hottest day of the year.) It would also open the door for another set of poles and another set of wires running down every street; one set per competitor. NYC was like that in the 1890s, and some places in Asia are like that today with hundreds of wires on every pole and laying over every rooftop.
But a death spiral in which rising rates paid by the remaining non-solar customers drive more and more customers to generate their own power could still be possible. But it would not directly affect utility profits as the TFA claims. The regulated utility business model would be challenged, not the profits of utilities that remain regulated. Those profits are guaranteed by law.
We should also recognize that lots of the population lives in high rise apartments and do not own enough rooftop or yard square feet to use solar panels.