I absolutely understand the problem of ever-increasing time to reach newer content. However, I feel that, for a long-term solution, making the progression up to that new content more streamlined and engaging is overall more effective. In addition, prompting a purchase to skip content is extremely risky, as it gives off the impression that the content you can skip is not worth playing.
I think for a short-term solution, allowing players that have yet to unlock the new content the ability to play the new stuff is not a bad thing. Maybe allowing players to play the new content, bringing in their current equipment or loaner equipment, but not being able to bring the new content outside into the main game until they have unlocked it like they would without skipping could work (i.e., weapons/warframes found and obtained via Whispers tileset/quest would remain locked from the main game until the standard way of unlocking the content has been achieved). In theory, that would allow people to jump right into the new content, but have the main game always available to experience the intended narrative.
I know that the big expansions are what brings in new players, and I've always been interested to see if one of those big expansions could be a retelling of the old Warframe story that is lost to time from limited events. If, for example, we could retell old events as a form of campaign as the player works through the star chart with the modern-day quality we see in newer updates, there could be a much more coherent journey through the initial star chart, into the Sentient storyline, and now into the newer content.
There's a whole discussion to be had looking at a narrative side, where giving the Grineer/Corpus time to breathe as proper antagonist factions within the story can allow the later antagonists of the Sentients and Wally (Infested could also fit as a higher-level antagonist to the Grineer/Corpus) to have that much needed impact of a larger threat appearing that overshadows the previous antagonists (Grineer/Corpus > Infested > Sentient Threat > Void God).
I'm curious if a "Warframe Retold" update would be on the table, to not only retell the pre-Sentient narrative, but retell it in a modern, polished way that offers a proper onboarding experience for new players, and a new perspective on older & missing lore for older players (Alad V's story is still out of order). I think this would be a massive step forward for both the long standing issue of the new player experience, as well as remove massive downtimes between narrative segments in the early planets, helping that new content arrive with less "slog" in between.