It's not (just) you. Yamaha themselves has discussed, in the forum, the contribution a keyboard feel and even sturdy platform to place it on can have on the perception of sound.

I posted an idea in ideascale to request Yamaha make a MODX Retrofit Keyboard - more Robust, Resilient, better longevity. KB would be at cost to the user, a direct drop-in replacement, and only available for install at Yamaha Service Centres.

Another pschological factor, if you are aware that the keybed is flimsy/fragile* and can be easily broken with repeated hard playing, the user will tend to go "light" and not play with the same vigour they are used to.

*Based on numerous reports of broken keys (from normal, extended playing), from various forums over the last 2 years. Yamaha have replaced (like for like) keybed sections in those instances, but there is still a high possibility it could happen again.

I posted an idea in ideascale to request Yamaha make a MODX Retrofit Keyboard - more Robust, Resilient, better longevity. KB would be at cost to the user, a direct drop-in replacement, and only available for install at Yamaha Service Centres.

I doubt there would be enough interest to manufacture such a thing, unless maybe it became a new upgraded keyboard to be used in their boards going forward, so they were not making it just for this purpose, but rather making it retrofittable as long as they were going to be making it anyway. There's a related question as to whether there is any reason to believe they could come up with a design that most people would think is better, that was still lightweight enough to not make the overall board noticeably heavier (and also still within the range of what could be physically supported by the existing plastic chassis). (ETA: On the weight/chassis issues, I was thinking more of the hammer action MODX8. Seeing the OP was talking about the 7, that may be less of an issue.)

I doubt they'd offer an alternative aftermarket, but I think a better synth keybed is possible without being too heavy - I had the black version of the studiologic sledge which was light but had a good fatar (same as rev2 I think). The main issue is they need to differentiate sufficiently with montage.

Back on topic, it's maybe reassuring to know it's just not me anyway!

I think a better synth keybed is possible without being too heavy!

Yes, I edited my OP in light of the conversation being about the semi-weighted action. I think the action they have come up with for the YC61 is nicer than what's in the MODX7, and the board is still quite light. It might be nice to see a MODX6/7 rev with that action as an improvement, and sure, once the part exists, if it could be retrofitted into the existing models for a cost, that would be a nice perk. Though actions being so subjective, I imagine some people would say they prefer the current action anyway. There would also be the question as to whether that design could physically fit. At a minimum, I think the actual shape of those keys would have to be altered, because I don't think the MODX6/7 has the front clearance to allow a non-diving-board shaped key to be depressed. So I don't think they could re-purpose those keys into the existing chassis as they are, that would be an additional complication.

I wonder if the YC61 keybed has been used anywhere else at all - at least something close. I wonder because what's in MODX I have some general idea of long-term reliability from looking at other past keyboards. The YC61's reliability appears to be somewhat of an unknown (at least out in the wild).

If the YC61's keybed is more reliable then it's perhaps a good thing. On the other hand - there are those who think the YC61's keybed is too "spongy" or "wiggly" or -- I guess too much like the waterfall keybed it's meant to give some amount of reverence to. So at least those who wish their YC61 wasn't so "waterfally" wouldn't like the YC61's keybed inside MODX.

Out of the keybed tradeoffs - I think the synth action MODX6/7 tradeoff (vs Montage's 6/7 keybed) is slightly better than the MODX8 tradeoff. I lived with a MO synth action for a while (broken/repaired with superglue key an all) and didn't find it too difficult to deal with.

At any rate - the question was largely philosophical and I do think the perception noticed by the OP is correct. I would think the best way to adjust your perception would be to get either a complementary "bottom" keyboard with a more "substantial" keybed or use a MIDI controller that arrives at the same place. I'm not sure having yet another choice of tooth paste at the grocery store is going to help.

That said - it's been a while since there's been a rack version.