i think this is a grey area odd one, i used to sell Marin bikes, at this time they were Jon Whyte designed (like the model being talked about) we did sell them telling people lifetime warranty on the bearings, back then they were designed in the UK (by jon whyte via ATB sales) for the original owner of Marin in the USA, i slightly odd arrangement really as a flag waving US company having their UK importer designing the bikes.
now when i was selling them it was so rare to have to replace the bearings, they genuinely are very good bearings that they fit.
now the biggest issues i see with the lifetime warranty on the bearings as i see it is that not only are the bikes not designed here anymore, Marin the company was sold, they no longer work with ATB sales and the bike is 16 years old.
what i would tell you is that if it was ATB sales still i bet they would honor the warranty, i always found them good to deal with and very customer focused.
but the truth is now you are asking a company that is not the company the bike was sold by to honor a warranty on a bike they never designed and never had anything to do with and therefore won't have parts for.
are you being unreasonable? no if it was the original set up (ATB sales and original Marin owner) but sort of yes bearing in mind it's a different company that effectively you are asking for something free from that they had nothing to do with.
that said, most people will not know the company/importer changed so therefore it's not necessarily an out of order thing to expect them to warranty something if you didn't know the company effectively changed hands etc.
but to be honest as well, at 16 years i'd just buy the bearings.