the sh in shadow isn’t /s/ though, it’s /ʃ/. and I’m specifically claiming that no Spanish words start with s+hard consonant. s by itself is fine, for example sonriar obviously, but I claim that no Spanish word starts with ‘st’, ‘sp’, ‘sc’ etc. so you have estudiar, espalda, escuela. in Latin these were stūdium, spātula, schōla. Spanish added an e before the s specifically because it became hard for them to pronounce. this same shift happened in French, hence étude and ecòle, but not in Italian (studio and scuola.)

so I think you have it the wrong way around. the reason Spanish has those initial es in the first place is because it’s hard to pronounce consonant clusters without them.