That argument is similar to that against Japanese cars when they came to the UK in force in the 1970s. Parts expensive (which they were not), can't get service (cars were not complicated), etc. They said buy a Ford, Vauxhall, BMC, etc, as the local garage can fix it. The fact was Japanese cars rarely went wrong. Also, some Nissans had inside the BMC A series engine made under licence.
Parts are freely available for Naviens. Ring them and they will give a list of the service dealers. Men who know the boilers, not a jack of all trades. The hydroblock is the same as a Baxi.
Plumbase, a national chain,
of over 160 stores, with also online order, sell the Navien:
https://www.plumbase.co.uk/navien-boilers-navien-100010607-4000006
They also sell the Intergas range.
Many other dealers also sell these boilers.
Naviens service dealers, with most of England and South Wales covered.
https://navienuk.com/find-a-service-agent/
Look at the map, the dealer near Reading says a company called D Robinson. Is that Dan Robinson, the obnoxious one who was on this forum? Didn't he say the the same as you about Navien? LOL. He looked them up and became a dealer? LOL. Navien may have said get off the forum or no contract, as he would put anyone off any product. It is worth being on these forums then, as people like me inform of new quality products.
Owners are better with service contracts with the makers
who know their products, rather than a plumber down the road, who does a lot of drains and the odd boiler install, like one a year - expert on boilers eh?