Your experiment has good foundations, but this "experiment" is playing with our money. To offer these wards at an RP cost for a temporary rental is literally taking money from us for the purposes of data collection. The concept of ward skins is a great one, but if it is to be nothing but an experiment, remove the RP cost and sell them all for IP. At least with the RP concept out of the way, there is no actual money being sacrificed for the sake of experimenting on the playerbase.
This is the first time I have taken a serious issue with Riot in anything they have done. I have, to date, spent $300 on RP and I don't regret any of it at all. I just don't want to see LoL go the route of offering rental-only content. Ward skins are great and I would most certainly buy one of them if I could be assured of my purchase lasting over time. I may rent one for IP but the one I want (tomb angel) is an RP cost. While it is less than a quarter's worth of RP, it is still my money and if I spend my money I want it to be an investment, not a passing fancy.
I have tons of skins and rune pages that I have purchased with RP, and I still have access to all of them. This includes Snowmerdinger and the Legacy skins I purchased during the sale; I am not opposed to limited purchase windows, but I am opposed to limited duration of said purchases. If my Snowmerdinger was temporary I never would have bought it. I want my limited edition skins/ward skins/etc to build value over time, not last for a certain length and then disappear. I purchased Snowmerdinger because I knew he would be unavailable for purchase in the future, but still available for use.
RP is for long-term investments. Yes, RP can pay for boosts, but even boosts have a per-win basis. Making these ward skins last for only one week per purchase (starting from when the purchase is made) really cuts into the value. A limited purchase window stacked on top of a limited duration means that this entire concept has a shelf life of only a few weeks. After those few weeks (even if it is 52 weeks) the money spent and IP invested will count for nothing. Nobody wants content that they can't control in any way. While we only technically "rent" skins until we're banned or Riot sees fit to shut down their servers, it is at least with the assurance that we will have them for as long as we deserve them.
I know your post said that we shouldn't rant about rentals, but I really want to establish that, as a heavy investor who has never seen reason to complain before, I will NOT approve of rental content if there is no permanent option alongside it. Rentals are fine; rental-ONLY is not.