Some truth in that and I share your concern regarding the apparent low numbers of emerging Scottish youth in Scotland/England's top Leagues.
But your picture dismisses those youngsters that were born outside of Scotland but qualified for Scotland through having at least one Scottish parent. So the "grannie" comment is a bit misleading. If you look to those who qualify via parents, then Tierney, McTominay, Gunn, Brown, Dykes and, closer to home, Max Johnston should be regarded as having successfully come through youth systems. Both here and elsewhere. Or do you think having Scottish parents is not enough to be regarded as "Scottish" youth?
Hickey, Hendry, Patterson, Gilmour and Ferguson are products of the Scottish youth system and comparatively recent additions to the squad.. You can add McGregor, Robertson, Armstrong, Christie, McLean, Souttar, Kelly and Clark if you care to go back further. All in the current squad. And the McCrorie brothers who may yet break through to the Senior Scottish side. Might not be France or England quality but more than good enough for a place in our squad in their prime.
And looking at the U21s, the two players who posters on here thought excelled in the last few days were Ben Doak and Kieran Bowie. Both born in Scotland and progressing well through English youth set ups. So, again, the Grannie reference does not apply. There are likely more from that U21 team but I cannot be bothered checking.
We can always do with more talent coming through, preferably Scottish born and bred, but there has been a good few in recent years and I don't believe the situation is anywhere near as dire as you suggest. And if Clarke delves deeper to find those who qualify via grandparents, then good luck to him. He would be negligent to ignore any legitimate source open to him. Off the top of my head, only Cooper and Adams are grand parent additions currently. Hardly the majority. The problem might be getting the Manager to ease out senior players to allow scope for youngsters to be introduced.