The problem with the Welsh was that they were always far too tolerant of their English neighbours. The Scots resisted English colonialism far more successfully and are, subsequently, far more developed as a potential independent nation.
If you go back into the dim and distant past - by which I mean the last Ice age - the inhabitants of what is now modern Wales originated in Central and Southern Europe, and it was these hunter-gatherers who made their homes in Wales (and the rest of the UK). At that time, however, the UK and Ireland were a peninsula of Europe, and it wasn't until some five or sic thousand years later that the Irish sea and the English channel formed.
Moving forward a few thousand years and the Romans arrive on the scene, happily subjugating anyone and anything that stopped them building roads - such as the A5. Prior to their arrival, however, we know that what is now Wales was assimilating immigrants from all over Europe and exchanging ideas with Saxons and others to learn new skills in Bronze work, jewellery crafting and the technologies of the time. Five tribes evolved which divided Wales between them, and what's interesting to note is that the origins of these tribes are thought to have been Central Europe, and - although there's significant scholarly debate about this - it appears that the tribe holding Central and North Wales (the Ordovices) originated from Romania. So, Hugo, you probably have some long-lost ancestor living in Eastern Europe :-)
The Romans, not being the type to pat heads gently, beat the living daylights out of the tribes in the North - mainly because they saw the value in holding Holyhead - and built Llanrwst in passing, plus a nice stretch of sea-front track in Colwyn Bay. So, by 400AD, we can assume that most of who we call Welsh were a mixture of Romanian and Italian descent.
(It’s taking a while to make the point I’m trying to make, but bear with me). The next significant phase came when the Plantagenets took the English Throne (through murdering the competition) and we all know that they were - essentially - French. It was they who built Conwy Castle and here we finally get to my first point: much as revisionists might like to suggest that the ex-Romanians and ex-Italians who comprised the Welsh at that time were simply too tolerant and happy, it seems that it was the educated Welsh - the top tier, if you like - of the Welsh themselves who decided that they wanted in at the Royal court and started a pattern which - ironically - has persisted to this day. They left Wales and moved to the Home counties, and the Statute of Pleading in 1362 established English as the language of the Court, where the Welsh there fell over themselves in the rush to learn and use it.
Now to the second point: it’s not that the Scots have “resisted English colonialism far more successfully”; it’s simply that - historically - Scotland is a country which few have ever wanted. The Romans did eventually get to the North of Scotland but a combination of Midges, weather, temperatures and extremely difficult terrain led them to concentrate efforts on the South of the country. During the Middle ages, the various tribes fought with each other endlessly, until eventually the Picts, from Ireland, became the ascendant tribe.
In the 12th Century, the Davidian revolution, combined with an influx of French and Germans (probably one of the few cultures acclimatised to the Scottish weather) combined with the Norse tribes that originally held the Orkneys led to a pleasantly peaceful time for the Scots, until Alexander III popped his clogs, and Scotland’s ruling elite decided to make a treaty with France, to avoid having to fight them in Edward’s army.
That didn’t stop them all fighting among themselves, however, but Scotland was ready to reap the rewards of immigration and passed the Education act of 1486, which made Scotland the first country since Sparta in classical Greece to implement a system of general public education. No wonder they produced the world’s best engineers, then. What cut short Scotland's proud independence, ironically, was the near bankrupting of the Scottish landowners when they became involved in a get-rich-quick scheme in Panama in 1698. It was then easier and safer fro them to enter a union with England.
What all that goes to show is this dewey-eyed regard some have for what they consider ‘their country’ and ‘their language’ is actually more the result of a hotch-potch of immigration, cross marrying, endless fighting, murdering and torching and bad financial decisions. We are - all of us - from Central Europe; and, going back even further, probably from India (Pakistan). Where we;re born, and where we grow up is simply an accident of serendipity.