if they are large enough and have components that need to scale independently than a bottleneck in one of those components may affect availability of the service.
Or may not. I design and create products that serve particular problem for customers while satisfying reasonable constraints. If it comes to a situation you've described than it will be spun into a separate component. I already told that I did and know how to do distributed applications. The keywords here are "WHEN NEEDED". And on modern hardware it is not in my cases.
