Re: When do you replace?
The 100D reached EOO(end of order) on 2018-07-26, then 2022-07-26 is the LSE(last service extention) then 2023-07-26 is EOS(end of service). So you can renew the 1yr support a couple of more times.
On the other hand, 6.2 or later OS doesn't support 100D due to unsupported ASIC NP4Lite. So 6.0.x is the last version of OS on the unit.
I would expect FTNT would keep update 6.0.x when major vulnerabilities are found and need to be patched at least until the EOS date due to the fact no options to go up to 6.2 or above.
My personal opinion is it's completely up to the situations if the current 100D with 6.0.x is well enough and comfortable for your needs. If that's the case, why would you spend money now while you can save it for next or the following fiscal year? But, if you want to have a better performance with new feature sets and want to explore them, going to "F" version would provide you lots of opportunities.
If you can't decide now, I would just renew it for one more year then, secure the budget for next year well ahead, which actually we decided to do for 200Ds.