I think in 5 years we won't even be having the conversation because 90% of cruising boats will have LFPs.
The basic premise is correct here. As a flyer for 25 years, I saw analogous technological and economic evolutions in avionics (e.g., GPS navigation and GPS IFR approaches). Acceptance accelerated exponentially as applications improved, we all caught up with the technology, and costs came down.
With lithium batteries, we're seeing a similar ramp-up of acceptance. I remember reading in Popular Mechanics about DIYers building Li batteries. Yikes! Then innovative battery manufacturers jumped into the market, advancing the technology and battery access. Significant complexities and safety issues remained (e.g., fires!) preventing broad-based acceptance for many of us. Further modifications and improvements were made. Fast forward to LiFePo4 and other technologies, which have eliminated the previous safety issues and the improved technologies (e.g., internal and external BMS, self-balancing, etc.) making them easier to understand and use.
In terms of costs, early on, the lithium battery installation market was a huge cash cow for marine electricians who followed and understood related AC/DC applications, including solar. I remember just two years ago getting a quote to install a lithium house battery bank for $30,000!! I didn't get it, but wow, this thing was complex with lots of components, total rewirings, etc. -- I'm not an electrician or a tech of any kind, but this was probably way more complex than it needed to be to justify the electrician's time.
For most of us, those massively complex and expensive systems are not needed or justified to get lithium batteries on our boats (to the chagrin of electricians). Now we have Smart Drop-In LiFePO4 batteries, backward compatible with many existing inverters, chargers, and battery monitor systems already on board. Costs are getting closer to AGMs. (By the way, we should be comparing "total useable AHs," not just 100h AGMs to 100ah Li battery.)
In any case, the "Guy with a Boat" is absolutely correct that in five years, we won't be having these discussions as we all catch up with the technology, applications evolve and improve, and costs come down.