I must admit I have wondered this in the case of a long term acquaintance (someone I spent a lot of time with for several years when our older children were babies and toddlers because our lives coincided a lot) who made it very clear she was only just coping with 2 small DC and couldn't go out with them both by herself unless she knew the destination would be easy (a known toddlers group or fellow toddler parent's house) and there'd be people who could and would help at her destination ... That's fair enough as we don't really know what we're getting into sometimes, especially conceiving DC2 while DC1 is a small immobile baby and thinking its all going to get easier quickly before realising perhaps you're someone who struggles with toddlers...
I thought wtf when she excitedly announced that she and her husband had decided to start to ttc DC3 immediately, because she never stopped telling us she couldn't really cope with the two she had and couldn't go out with them nor get them to bed nor much else without help.
She was a perfectly competent human who'd had a very good career but given it up to be a sahm and seemed to really struggle and be so worn down by it and I honestly thought wtf are you doing prolonging it and taking on more of the exact thing you keep telling us you can't manage? Why do you think this is right for you?
I didn't say anything obviously. I couldn't really bring myself to congratulate her on deciding to ttc though (some people did) I just tried to fade into the background! An unplanned pregnancy would have been different as you make the best of it if you decide to go ahead, but planned ttc in that situation... 
Oh well.
As others say situations can change, financially, health wise etc. People who could cope can stop being able to. People also find different stages easier or harder -for some people babies are easy and toddlers difficult, for some primary age is more difficult than toddlers or teens... So you don't totally know who'll be coping best in five years time.
Still taking on more when already struggling badly with what you have does seem a poor decision when deliberately planned, on the surface anyway!