Tottenham Hotspur exists in a cycle of windows. Windows of opportunity and windows of rebuilding. Opportunity to win things, and periods of reconstructing its core once its best players have been bought away. That’s why every season now for Spurs, currently at the top of such a cycle, feels urgent. It may be a title contender this year, but there are never any guarantees for the next one.

Lately, Tottenham has done a remarkable job of retaining its top talent. The players have bought into manager Mauricio Pochettino and his project. They understand that he has them playing better soccer collectively than the sum of their parts otherwise probably would under most other managers. They thrive in North London, and there are never any guarantees that they would elsewhere. But how long will it last?

Even after Spurs move into their new stadium next season, and their revenue shoots up even further, a wealth gap will likely remain with the Premier League‘s truly big clubs. Chairman Daniel Levy is notoriously tight with the purse strings, and chances are he will never pay the salaries available at Manchester City and United or Chelsea. Which, logically, puts an expiration date on every star player’s tenure with Spurs.

This, as you might have gathered by now, is about Harry Kane. The 24-year-old striker has probably been the best striker in the Premier League since the start of last season — well, since September, because Kane is a famously slow starter and struggles in August. Kane has even slipped into the conversation about whether he is the best striker in the world at the moment for his uncanny scoring ability and all-round play. He finishes, and he creates as well.

In the 2017 calendar year, his 29 goals trail only Lionel Messi in Europe’s five biggest leagues. With eight goals in nine Premier League games so far this season, Kane is likely to breach the 20 league goal barrier for a remarkable fourth year in a row, all before the age of 25. Now there’s talk about Real Madrid coming after him next summer as its latest galáctico signing, the ultimate vindication of your superstardom.

In a sense, Spurs’ ability to retain Kane, who has been at the club since he was 11, although he was sent out on loan four times early in his pro career, is a litmus test. Keeping their best players and building that lavish new stadium are supposed to pave the way for Spurs to ascend from the upper-middle class of English soccer teams into the aristocracy. To contend for prizes every year, rather than a few times a decade.

But in order to accomplish that, they need to keep hold of the Harry Kanes. And they need to figure out how to keep Real from adding him to the list of big stars Spurs sent that way, including Gareth Bale and Luka Modric, who cemented their status as some of the top players of their generation not in North London but in the Spanish capital.

If next season Kane plays in the white of Spurs, rather than that of Real, it will say as much about the club as his loyalty. If the offer is sufficiently big, and the upgrade in title prospects large enough, players will always go in the end.

On Sunday, Kane only added fuel to that fire by scoring twice and adding an assist in Tottenham’s comprehensive 4-1 defeat of Liverpool at Wembley Stadium, its temporary home where the club has struggled to establish the kind of superiority it had at White Hart Lane. It was the first time in 10 games that Spurs had managed to beat Liverpool, whose title hopes are sinking without a trace after a 3-2-4 start. Tottenham, meanwhile, moved into a tie for second place with Manchester United, five points adrift of City.

In just the fifth minute, Kane beat Simon Mignolet to a ball dumped in between them, evaded the goalie and defender Joel Matip and dinked the ball into the empty net.

Some seven minutes later, Heung-Min Son applied a tidy finish on a perfect set-up pass from Kane.

Liverpool briefly asserted itself in the first half. And when Spurs goalkeeper Hugo Lloris hesitated on a ball behind his line and stayed home, Mohammed Salah capitalized, beating the defense to the ball and bouncing his finish off the inside of the far post.

While Liverpool had most of the ball, pressing high and hard, the defensive mistakes on Spurs breaks were costly. Deep in first-half injury time, Dele Alli whacked home a poorly cleared free kick from outside the box to make it 3-1 and kill the game off.

Early in the second half, Kane got his brace when he knocked in a rebound with a volley from close range.

It was the sort of performance from Kane that makes the prospect of a future without him terrifying to Spurs fans — to say nothing of those within the club itself. Because, for a club like Spurs, he would be irreplaceable. There are only a handful of strikers of his caliber even around, if that, and none would be available to the Londoners. Even if they scraped the money together, it’s hard to tempt such a player to a club that might be a title contender some years, but that will likely never quite be a title favorite.

Which is to say that Spurs will probably be as good as they’ve been lately for as long as Kane is around. Whether it be for 10 more months, or 10 more years.

Leander Schaerlaeckens is a Yahoo Sports soccer columnist and a sports communication lecturer at Marist College. Follow him on Twitter @LeanderAlphabet.