The players who have patience and understand their late teens/early 20s are all about maximising any opportunity, going through the grind, being patient and learning will make it. Guys who chase after the big buck and stardom will fall away in the hype.
took Harry Kane 4 years and 4 loans to get a PL appearance for Spurs. In fact let's look at the last England squad.
Heaton - At United 7 years, no appearances. 6 loans away before dropping down to the Championship with Cardiff, Bristol City then getting promoted with Burnley.
Butland - 3 years at Brum before loan to Cheltenham, then a full season in the Championship with Birmingham before coming into Stoke, where he played 12 times in 2 years before coming first choice, with 3 loans to the Championship in that spell.
Forster - 6 years at Newcastle, couldn't break through. 3 loans in England before 2 years on loan at Celtic before moving there permanently. Only after 4 seasons as first choice in the SPL did he get a move to the PL and Southampton.
Hart - 3 years of playing at Shrewsbury, 1 year first choice in League 2. Comes to City, 2 loans in his first season, then gets regular game time. Then we move up a level squad-wise and he goes to Birmingham and stars as first choice, then comes back and becomes number 1.
Trippier - Starts with us, no games. Has 2 spells on loan at Barnsley before moving to Burnley in the Championship. Spends 4 years there, gets promoted and goes to Spurs.
Bertrand - 8 years a squad player at Chelsea, 7 loans before he got a permanent move to Southampton.
Jones - A rare case of break into the first team at a PL club (Blackburn), then get a big money move in 2 years to United. Hasn't fulfilled the early promise.
Cahill - 5 years at Villa, 2 loans and 1 season where he played 20 games. Moved to Bolton and after his first season became a regular, then moved to Chelsea.
Stones - 3 seasons at Barnsley where only his last was as a regular. Moved to Everton and claimed his spot and went on to us.
Walker - Briefly at Sheff Utd before being bought by Spurs, however 3 loans followed over the next 2 seasons before he cemented his spot at RB. Then moved to us.
Cresswell - 3 years at Tranmere, 3 years at Ipswich then a move to West Ham.
Getting bored of proving my point so a few more...
Smalling - Maidstone - Fulham - United.
Sterling - A rare exception.
Ox - A year in League 1 with Southampton before a gradual introduction at Arsenal.
Alli - 4 years in League 1 at MK Dons before Spurs.
Dier - No apperances at Everton, infrequent appearances for Sporting before move to Spurs.
Defoe - Loan to 2nd Division Bournemouth before being given a chance at West Ham.
Rashford - The 2nd rare exception.
Lingard - 4 loans away before becoming a squad player at United.
Kane - 4 loans over 3 seasons, scoring 16 goals in 3 years. First PL game in 2012/13, 10 league games in 2013/14, only scored 4 goals. Took till 2014/15 to be a regular and hit, 5 years after joining Spurs and at 21 years old.
In summary, if Sancho isn't prepared to go out on loan, even down the levels, and be patient and realise unless he's a very rare exception he won't break through into a top team in even a squad role until his early 20s, then he'll throw his career down the pan being greedy and going for broke too early.
Sign the contract, do the time and earn your stripes. Be wise.