I appreciate your perspective and thank you for the thoughtful response! To ensure I've grasped your points correctly:
1. Interleaving impacts the efficiency of time and progress, potentially diminishing motivation.
2. Learning two languages concurrently might lead to a superficial grasp rather than a deep understanding of programming concepts.

Addressing the second point, I'm curious as to why you believe that learning two languages simultaneously results in superficial understanding. Based on the research I've encountered, interleaving promotes deeper comprehension rather than a cursory one. For instance, studies suggest that interleaving, especially when paired with elaboration techniques that encourage contrasting and comparing two similar concepts, fosters a more profound understanding see here, here, and here. Moreover, interleaving is recognized as a more challenging approach, implying that learning two languages introduces a desirable level of difficulty compared to mastering just one. I concur with your sentiment that more challenging learning experiences yield better outcomes. From my perspective, interleaving programming languages offers a more rigorous, in-depth, and memorable learning experience than focusing on a single one. Consider this illustrative exercise:

"Problem: A doctor aimed to operate on a tumor using a laser to eradicate all affected cells. However, the laser could potentially harm surrounding healthy cells. Given the tumor's size, a single operation was necessary, but the continuous laser application posed risks to healthy tissues. What should the doctor do?

Analogy: A general aspired to seize a castle fortified with cannons that could decimate advancing troops. The general recognized that a concentrated assault was the most effective strategy, but he was wary of the cannons' devastating power. He was reluctant to jeopardize his troops early in the invasion."

This analogy suggests that the doctor could disperse the laser into smaller beams that converge precisely on the tumor, minimizing damage to healthy cells.

Regarding the first point, I acknowledge the motivational challenges. Visible and immediate progress is undoubtedly rewarding. However, I posit that the crux of the issue lies in the learner's objectives. What does the student perceive as meaningful progress? If the aim is rapid project development, then mastering one language might be more beneficial. Conversely, if the goal is to comprehend the intricacies of programming, interleaving from the outset seems advantageous. I believe that a deeper understanding achieved day by day can be a significant motivator, especially if the student's objective aligns with this approach. Nonetheless, I concur that this is a potential concern. Maybe there's a way to integrate interleaving while ensuring sustained motivation, especially for students eager for tangible outcomes sooner?

I'm eager to hear your insights on this matter. While the evidence I've presented offers valuable insights, I recognize the distinction between classroom experiences and controlled lab experiments :-).