The 5 year old console argument makes no sense. PS1, PSP, PS3 & PS4 all peaked in year 4 or 5, it's normal for PS consoles to do their best around this time.
i think the better argument would be pricing, ps4 is 5 years old and 299$ which is the same price as the switch. the fact that ps4 is selling this good at this price after 5 years is the amazing part because if it had xbox like deals which what the norm the late in the gen, it would have destroyed the competition.
No, pricing argument dont make sense also too much also because they are not same devices, don't have some power difference and capabilities. PS4 selling so good so long is relly amazing and we talking about console that will become second best selling home console ever, but there is no reason to downplay Switch success ("PS4 is five years old more or less, it should not even be close"), actualy fact that Switch in its second year is selling similar even better with one of bests years of console that will become second best selling console ever, tells us how much impressive Switch sales are, especially when Switch is still selling without any official bundles at cheaper price point with launch price and without big game until now this year.