(I don't 100% buy this myself, just challenging the premise)
pro-lasting: there are plenty of places with buildings that are hundreds of years old that are perfectly fine and useful buildings and there's no really compelling reason to tear them down and rebuild them.
Another argument is that the longer stuff lasts the less you may need to maintain it. Given that maintenance costs are hard to figure out and it's hard to get dedicated budget to keep things going (the present problem in the U.S. w/r to infrastructure for example), it's unlikely that things will be well maintained throughout their potentially useful life. If the stuff just lasts better, it's more likely to have a longer useful life...which is simply cheaper.
pro-replacing: An awful lot of national GDP in many countries comes from construction projects. Keeping a constant turnover in infrastructure and housing keeps huge populations employed with decent paying jobs.
Cities don't always have an infinite utility. The U.S. Rust Belt is an example of many cities that may have outlived their intended utility. This means that populations will migrate to cities with more current utility and the need to rearrange those areas to suit the change in population is important. A fixed or stagnate inner core, that could be replaced with higher density housing, is preferable than simply sprawling out elsewhere.