By the end of the 2014/15 season Willis was still 20 years old though and had made 74 apps. He didn't turn 21 until August 2015 - which was the start of the 2015/16 season where he suffered his injury and only made 4 apps - which brought him up to 79ish in total. So, I couldn't care less what he did beyond the 2014/15 season because it's irrelevant to my point as that's past Thompson's current age bracket of 19/20.
But if you align Thompson and Willis' career at their exact ages then I understand your point but like you said Willis gained more game time that Thompson - which still supports my point. But what you're failing to consider is that Willis was a mainstay in the side where he played more or less week in week out with the exception of injuries/rest - hence by the end of the 2014/15 season he'd amassed nearly 75 apps. Plus he had the experience to push on in the 2014/15 campaign to be a mainstay in the side all season by making a further 40 apps on top of his 30-odd to bring him up to that total figure. He wasn't playing the odd game in the JPT. Thompson on the other hand is fourth choice behind 3 hugely talented CBs, all of which who could command a starting spot. Don't forget Thompson isn't far off being 20 so unless we're seriously unlucky with injuries then he isn't going to get anywhere near the game time Willis received at Thompson's age between now and the end of the season. Hence my point that a short-term loan would be a preferential option.
I'm not saying he may as well retire now due to his lack of first team action as a lot can happen in a year though so maybe Thompson will break though. But there is a point can be made that there's a risk of him falling behind other graduates of a similar age. Look at Kyle Finn as a case in point. No game time at CCFC but once loaned out he flourished.
I get that but if you are going to discount the experience Thompson has gained, then you ahve remove all of that from Willis too- All of his 3 apperancves in his first season were substitute ones- so minus 3
His second season- three appearances again all substitute. so minus 3
3rd season- he made 23 full appearances including all cups. About 19 without cups. . This is the point Thompson is at in his career. 19 turning 20.
4th season- so as you say when he is 20 turning 21. - is the first season he was a regular first choice. Making 40 appearances
You aren't using a fair assessment, if you discount subs appearances for one you have to do the same for Willis. I agree that Willis had more exposure, but almost the entirety of his first 4 seasons with bus was at RB, not CB like Thompson.
Thompson was loaned out last season and impressed. He impressed in the Checkatrade, and did well against Portsmouth when he came on. However your point about him not getting first team exposure like Willis is invalid, because Willis only got his chance due to injury and lack of form, so Thompson has to be given that chance too imo. Plus if you want to compare the two you have to give him until the end of this season to compare career stats.
I don't think there is a better 4th choice out there, last season proved to us we needed 4 CB's, and I am certain he will be needed at some point.