I would not agree with you on that.Those considering vintage KB or cheap CASIO, etc, consider 40W or more bass amplifier ASAP.
The stock speakers, especially 30 years later, could potentially turn off a new player.
A keyboard has a dynamic range far greater than either a Bass or standard Guitar and they do require amplification that is designed to cater for the frequency range of a keyboard.
Reasonably sized Powered Speakers is IMO the way to go for home use, as long as there is enough headroom in the power of the Speakers.
5“ or 8” kit by PreSonus, JBL etc are reasonably priced kit even a pair of PreSonus Eris 4 speakers at about £80 will sound vastly better than the inbuilt speakers in a Casio or Numa Compact or something similar etc. (the type of keyboard with low powered inbuilt amp and small budget speakers).
My own are a matched pair of Active Monitor Yamaha HS5 which suit me just fine, each 80W and cost £330 for the pair.