I've been building software since my university days back in the 80s. I've been running my own consulting and software company since 1994, mostly dealing with very large complex control and optimization systems.

Agile might be good if you have a small simple system. The bigger and/or more complex the system, the more time and money you will save long term by concentrating on getting the design rigt rather than concentrating on getting a product out the door first.

I understand there are markets where agile is a requirement to stay in business short term. IMHO you are setting yourself up for long term failure.

We're currently updating an old system where the one previous vendor designed and installed the system and a second did an upgrade about 15 years ago. Both companies are no longer in this business area. Looking at their code I see why. They must have spent huge sums on support until management said "we're closing down this business area." Good for us. :-)