Most assistant managers have shown they can't go from being a number 2. Shakespeare, Phelan, and plenty of others have not made a success of a switch from the shadows, to the spotlight and the buttons. Appointments like Alan Shearer were not serious, he didn't have more than coaching experience.
No manager is really a long term appointment these days and there are no guarantees of success. Plenty of other teams abroad have to not just recruit from their countrymen, but develop their own managers. But we've not had a quality young manager waiting in the wings before and while it's not really common for PL, it's quite successful elsewhere.
Unsworth has very impressive managerial experience, albeit with under 23s and a cameo with first team. But he's 7 years in coaching and 3 of those as a manager and under several managers and styles. Moyes, Martinez, Koeman all came with something different he learned from, but none of them played for the club or started with a deep understanding.
It's not always the best players who make the best managers Wenger, Ferguson, Mourinho etc. Guardiola came through youth team, Zidane too, Martinez, Howe, took over at familiar club, even Koeman was most successful at Ajax. The average age of managers in the Bundesliga is probably 12 by now. Maybe other PL clubs might be missing a trick by buying off the peg, rather than developing in house with intent.