Building alternatives to capitalism is not just about eroding it, but also abut developing a working alternative to capitalism, which can be scaled up in case of a revolution. A lot of people just look at the bad parts of capitalism and forget that there are even worse options. Revolutions just change the balance between already existing forces in a country. So without a working alternative, they just end up looking like the old system with different branding. There are a lot of “socialist” countries like that in history.
The other part of it, is to increase the anti capitalist movements power. Basically people believe you, when they see that what you do is working for them. This article explains this fairly well.
Other then that we can implement degrowth ideas within a capitalist framework. EU emissions trading for example is massively neoliberal in its approach, but it is also a clear way of limiting environmental damage by limiting fossil fuel consumption. Also things like UBI are clearly possible, as most Nordic countries already have social welfare systems, which are more limited, but certainly have some similarities.