It's worth looking at historical cases of how workfare programs displace public-sector union workers and suppress wage, even when the proponents initially claim this won't happen. The experience in NYC under Guiliani is a good example of how this plays out.

Guiliani takes office in 1994 on the back of a campaign promising to cut the budget and reduce staff sizes. In 8 months he cuts 7,000 staff from the payroll. Some Departments indicate they intend to use his workfare program to offset some of these losses.

https://www.nytimes.com/1994/09/05/nyregion/giuliani-job-cuts-starting-to-have-impact-on-service.html?searchResultPosition=25

While initially skeptical, District Council 37 (the municipal union) agrees to a pilot version of the workfare program. This requires people to work in schools, as crossing guards, in cafeterias etc.

https://www.nytimes.com/1994/12/15/nyregion/first-accord-by-new-york-with-a-union-on-workfare.html?searchResultPosition=48

Less than a year later, the workfare employees become critical to making up for the shortfall in public sector cutbacks.

https://www.nytimes.com/1995/09/01/nyregion/new-york-workfare-expansion-fuels-debate.html?searchResultPosition=5

The unions rightly become more skeptical about what role this program is playing. Is it additional services, or are they replacing permanent union workers at a lower rate?

https://www.nytimes.com/1995/09/01/nyregion/new-york-workfare-expansion-fuels-debate.html?searchResultPosition=5

Despite claims that the program will give its participants dignity and assist them in moving to paid employment, few participants find any of these benefits.

https://www.nytimes.com/1996/08/13/nyregion/new-york-girding-for-surge-in-workfare-jobs.html?searchResultPosition=16

A year later, the Transport Workers Union agrees to 500 job cuts and to let welfare recipients clean subways and buses. They do so under the threat from the transit authority to hire a private company that would replace 2,000 workers.

https://www.nytimes.com/1996/09/19/nyregion/transit-union-agrees-to-allow-workfare-plan.html?searchResultPosition=19

Stanley Hill, head of the municipal workers union, starts to doubt the decision to allow workfare alongside union jobs and calls for a halt to the program. Guiliani proceeds regardless. The expansion of the program further undermines workers' wages.

https://www.nytimes.com/1996/09/23/nyregion/new-york-union-leader-urges-halt-to-broadening-workfare.html?searchResultPosition=27

Despite the no-layoff clauses, many union workers feel threatened that their jobs will be easily replaced by workfare participants that are working alongside them.

https://www.nytimes.com/1996/10/01/nyregion/union-head-is-assailed-on-workfare-concessions.html?searchResultPosition=39

Despite Guiliani's commitments, workfare participants end up replacing union workers. They perform the same tasks for roughly a quarter of the pay.

https://www.nytimes.com/1999/02/04/nyregion/union-to-sue-giuliani-administration-over-use-of-welfare-recipients-in-jobs.html?searchResultPosition=34

We could go on but the playbook here is incredibly clear:
1. Gut public services
2. Force welfare recipients to work for sub. standard wages
3. Use the threat of workfare to suppress wage demands
4. Replace union workers with workfare to make up for the decline in services

This example shows what happens when you pay people sub-union wages to do similar work. All the assurances are made at the start. It all accounts for nothing. Workers are left worse off. And this in NYC, in workplaces that are unionised and organised.