A monolith makes developing initially a lot easier. Over 15 years though, you are bound to have developers of various calibre leave their mark on it. Many don't even understand good modelling and inevitably drive an otherwise healthy monolith with well defined boundaries into a soup of couplings between domain concepts that should not know about each other. In theory though it is totally possible to have micro systems within the same monolith code if you model things that way.
Eventually, your team will flip the table and start thinking how to avoid the problems they are having with the monolith and decide to do down the micro-services way. In most situations developers are likely to face a lack of commitment from the business to spend time/money on a technical problem they do not understand but have to support the development of for a lengthy period of time. Most developers will compromise by building pseudo micro-services without their own databases which send requests to the monolith where the core business logic had to stay.
The benefit of micro-services IMO is driven from being able to separate business domains in a way that keeps each responsibility simple to understand for a new comer and compact enough to not hide a great deal of complexity. It's worth saying this is a very hard thing to achieve and the average developer shop won't have the experience to be able to pull it off.
This is all to say, regardless of Monolith or Micro-services architecture, the key is experience and discipline and without a good amount of both in your team the outcome is unlikely to be a success over a long enough period of time. This is the curse of a business that lives long enough.