Higher demand on maxed out cards is a lot more important than most think, it increases the demand of everything, lower level cards, sell price and even the tokens and lower demand reduce everything as well.
There are several cards that his maxed out versions are cheaper to rent than lower levels, this happens with all kind of cards for most of the seasons, different rarities, summoners, etc , this is happening mainly because there are a lot less players playing on Diamond or higher leagues, Gold seams to be the best league for SpS earning, you need less Sps stacking is a lot easier and more important Sps earning are higher, yes the earning on Modern Gold I are higher than Modern Diamond III and Diamond II and for some part of Diamond I , you need to be almost at a maximum rating of Diamond 1 in order to earn more Sps, Here some stats of Sps earning:
Some changes should be made to the way Leagues are rewarding the players to encourage more going to higher leagues and not stay in Gold for ever, if there are no intentions to change it at least give a higher sps pool to Modern Diamond so the sps earning are not so low, take some of others pools to give to this one, the league that is causing the problem, the lower demand on maxed out cards.
Yes Land 1.5 should increases the demand on maxed out cards but I don´t think it will be too much or enough to solve this problem, it will not increases the demand on all cards, more likely it will do the most for high rarity golden cards, also no idea when will be released, so another solution could be implement before it if is possible.
Solutions:
Beside increasing the Modern Diamond Pool to make more players want to go to higher leagues increasing the demand of maxed out cards, another one will be:
Giving a bonus when you use a higher level card from your league: Similar than you get penalizes when you use a low level card you could have bonus like gold cards when you use a higher level card that your league allows, like using a level 8 Rer while you are in gold Leagues give you a 5% bonus, this way when players rent a card they will always check the ones with higher level and if the price is close they will rent this one, increasing the demand for them.
Modern Diamond League low rewards is a big problem too, even if the maxed out cards demand get solved.