From a customer’s prospective, how long should the manufacturer have replacement parts available?
Since we are talking industrial tools and not kitchen appliances, the answer is "as long as you are in business". The machines and their accessories aren't consumables. I can get replacement parts (at a horrendous price) even for many machines whose manufacturers are long out of business, since they sold their IP on to others.
Darex's current products are not significantly less expensive than Black Diamond drill grinders, and BD will rebuild their machines up to about 20 years old. I suggest that you really don't want to be (perceived to be) in the consumable/disposable machine business.
Obviously, "until the end of time" is not ideal from the manufacturer's perspective! But deviations should be thoughtfully considered, and the manufacturer should have a story to tell the customer for each specific machine/part that is dropped. "We don't want to mess with it anymore, and make higher margins selling new machines" may be true, but it's not compelling to your customer.
What percentage of the original machine price are you willing to pay in replacement parts before you look to purchase a new machine?
Over how long a time period? What is the new machine doing for me that the old one didn't? How much retraining do I need to operate the new machine up to the same level as the old one? How much new tooling do I have to buy to bring the new machine up to the same level as the old one?
How do you decide if the cost of a replacement part is too high?
That's hard to answer. There are at least three ways to judge "too high": based on original machine/part production price, based on one-off part price, and based on value to the customer. Inexperienced people may expect obsolete parts to be replaced at the original production cost. More experienced folks may have their own ideas about what a one-off "should" cost. And even if the customer avoids judgements about what a "reasonable" replacement cost is, only the customer can decide if the cost is worth it.
If I am sharpening 1 drill a month, I won't pay $200 for an M4 replacement chuck. If I am sharpening 100 drills a month with a variety of controlled point geometries and a new dedicated machine costs $1400 - $3600, I will. Both decisions are instant no-brainers. If an M4 replacement chuck cost $800, the second decision requires answers to the 2nd quoted question above.