The left-right divide ceased being a viable political distinction over the last thirty years in the US. And you are correct that this coincided with the decline and disappearance of conservatism in the form in which it had survived since the 18th century.
The old form of conservatism was the product of a landed gentry that had the resources to remain independent and felt the need to isolate itself from the newly enfranchised masses who would increasingly demand collective control over resources. At its base conservatism was reactionary although this could be justified on the basis of noblesse oblige. On the practical side the arguments for motivating the masses to self help and innovation was a moral argument hiding the self interest of the gentry and subsequent merchant class.
Steady urbanization with over half the population (in the US) living in cities removed the opportunity for the masses to emulate the landed gentry. The rugged individualism of the citizen farmer, the masses as a proto gentry, makes no sense in collective wage centered populations. Gentry became simply owners who had been lucky enough or innovative enough to gain capital through increasingly extreme capitalist systems. By its very nature capitalism seeks to eliminate competition. So pseudo conservatism became impossible to justify except as a status quo for the elite class. Those that have should have more. Those that don’t shouldn’t demand anything.
As you note the political parties became the route to power, i.e. wealth and that required protection for those who had gained power. Everyone is now an urban citizen but those with the benefit of previous wealth and power could focus only on two choices. These choices were the acquisition of more wealth through free opportunity to use existing wealth or exploitation and extraction of wealth from the wage earners. These positions were not in opposition. This is the merger of the two parties into neoliberal alliance over the last thirty years.
As conservatism has disappeared the political issues became social disputes over details that required increasingly arbitrary positions hiding the common goal of enriching the oligarchy that ruled both parties. This inevitably disintegrated into neo feudalism based on mindless support in exchange for tribal benefits, e.g. the ability to exploit specific tribal groups. This is very much a multilevel marketing approach where the early winners take nearly all and everyone else fights over exploitation of the relatively poor.