Those people couldn't be more wrong. Monoliths are not your friend. Deployment times will be longer, requirements for hosts where you deploy it will be bigger, you will end up with _huge_ instances to rent because some parts of you monolith want more RAM and some want more compute. You'll have to scale more than you really need to because some parts of your monolith are more scalable than another, etc, etc, etc.
You'll be unable to use different technologies to optimise that one particular use case, because it has to be deployed together with everything else and you totally don't have any infrastructure to call into another service. You'll be stuck with "one size fits all" technological choices for every component in your system.
Monolith is good basically only on PoC stage. Once you have business running, you need to dismantle it ASAP.
edit: grammar, spelling, formatting.