Having a spare printer or two still in their box is a shrewd way to maintain your hobby way into the future and or failing that a spare head would be a great asset to have too.
Most of the newer desktop Canon printers are just rubbish when it comes to build quality and are designed for throw away when trouble strikes and just get another one, because it’s usually cheaper than buying new heads for them..
The MX850 is of the PGI-5/CLI-8 generation and uses the same QY6-0075 print head as the iP4500, so the print engine in the MX850 is most likely an iP4500, a very good printer IMO. I think that some of the PGI-5/CLI-8 printers were among the best printers made for home use and iP4500 is among those best printers. Sadly they don't make them like that any more.
If you should buy a replacement print head is difficult to say. I think there is no true or general answer to that question, only different opinions.
When a Canon print head fails electronically it can damage the logic board and the damaged logic board can damage a new print head. So replacing a Canon print head is a gamble, and sadly I cannot tell the odds for success.
I think the question about replacing the print head must depend on what condition is the printer in apart from a failed print head? How did the print head fail? If it is severely clogged or suffers from cross contamination, it might not have failed electronically and chances of a new print head bringing the printer back to life is greater. And the most important question: What is the price of the print head compared to the value of the printer if it were OK and is the print head a new unused unit or a refurbished one?
Beware, some sellers from the far east sell used and defective print heads that have been cleaned externally and repackaged so they look like new and they claim the print heads are new and original ; original yes, new and OK no.
If your MX850 is in good condition with a low D-value and you can get a reasonably priced unused print head I recommend to buy it as a spare.
"Is the printhead the most failed part on these printers?"
Definitely YES
" I am wondering whether I should buy a spare printhead or just buy a spare printer? "
Whatever is less expensive.. many people in US seem to have bought a second pro-100, took the printhead out and put the "rest" on ebay.
BUT, as peterBJ has already said, there is no more new printer and no more new printhead available for your model.
Canon could easily produce some 1000 pcs, these would sell like hot cakes - but they don't... you know why..?
I just got a ip3000 (like new for 10€) for one reason: the QY6-0064 is still available, I bought two of them as spare parts
I doubt you will find reputable sellers.
Maybe you can find a printer (IP4500 , IP4500X , IP5300 , MP610 , MP810 , MX850) - from an old lady who never used it - on second hand sites.
This is an interesting thread. I, like many others, have suffered with printhead failures but have recouped the cost, many times over, by refilling rather than purchasing OEM carts.
However, reading grandad's post in another thread, about not letting an ink cartridge run to empty before refilling, makes me wonder whether this may have contributed to some printhead clogging I have experienced in the past. Certainly, in future, I will make sure that I refill before I get the empty warning.
Are Canon printheads more liable to failure than Epson or Brother printheads? Certainly they're easier to replace but do these other printers have more robust printheads? Anyone have any experience of these printers?