That's too "extensive" explanation i think.
Every season you have the chance to earn one additional stash tab. If you don't get it in S5, it just means that it'll take you one more season to get all the stash tabs (there's a total of four you can get by finishing the season journey's stash tab goal in four seasons). If you don't get your S5 stash tab it just means that you won't have the maximum of 4 additional stash tab (resulting in a total of 10) before season 9 (S6, S7, S8, and S9 for you). The earliest anyone will have all 10 stash tabs is season 8.
So to sum it up, as soon as season 5 is over (which is not for at least another 2 weeks, maybe even 3-4, we still haven't seen the announcement) you can't get the S5 stash tab anymore, but it only means it'll take you one more season; you don't really "lose" anything or "miss out" on anything. It's just delayed.
We don't now many seasons would be ahead. Maybe blizzard would give up on the "1 stash tab per season" idea completely next season. Or give up on seasons. Or maybe invent something else. Or maybe give us 5 stash tabs next season. We have no clue literally. So saying that "you don't muss out" your stash tab is incorrect i think.
If you haven't got your extra stash tab this season: you'll just have one less stash tab than the ones who got their stash tabs this season.