Disclaimer: I have a vested interest in the external waste ink approach to solving this so take with a pinch of salt...
That said... The SC-P400 is basically the same design as the R2880, R2000, R1900 printers. All of them have the same accessible waste ink system that can be redirected out into an external waste ink tank far more readily than going into the guts of the machine to replace the waste ink pads.
You do still need to reset the waste ink counter and this can be achieved using the WICReset tool which is basically a pay per reset system where you buy a "key" which you use to reset the counter and clear the error. Important to note that the reset does not freeze the counter, it just resets it back to its 'as new' state and then increments as before. So, if you buy/fit an external waste ink kit you will still be faced with needing to reset the waste ink counter at the end of each "life" when the waste counter hits 100% again.
My solution is the Printer Potty but other kits do exist and you can also DIY the fix yourself if you have the time to research and will to make it happen.
As to the printers lifespan... let's just put it like this... we have Stylus 900 owners whose printers are still going, erm... 15+ years down the line so yeah... it's still good for a bit longer yet.