The sensor in the Panasonic Lumix DMC-LX15 is 2.74X larger in area than the sensor in the Panasonic Lumix DMC-LX7. That is a considerable advantage.

If two sensors use the exact same technology, a sensor with 2.8X the area of the other would have an almost two-stop advantage in terms of signal-to-noise ratio (SNR or S/N ratio). By comparison, full frame sensors are approximately 1.5X larger than APS-C sensors and enjoy a roughly one-stop difference when they both use the same technology.

The pixels of the LX15 are slightly larger than the pixels of the LX7. This means that over an area 2.74X as large, the LX15 has 20 MP compared to the LX7 with 10 MP. While megapixels aren't the be-all end-all that some folks make them out to be, more megapixels allow larger display sizes before an image starts to look pixelated.

The 24-72mm (35mm equivalent ) f/1.4-2.8 lens of the LX15 compares fairly well to the 24-90mm (35mm equivalent) f/1.4-2.3 lens of the LX7. It doesn't zoom out quite as far, and it is a tad slower at 72mm (35mm equivalent) than the older camera is at 90mm (35mm equivalent), but only by about two-thirds of a stop.

Combine the differences between the sensor sizes and maximum aperture sizes of the two cameras and the newer LX15 has a 35mm equivalent aperture¹ of f/3.8-7.6 compared to the LX7 with a 35mm equivalent aperture¹ of f/6.4-10.6. This means that you can expect roughly a four-thirds-stop performance advantage at 24mm (35mm equivalent) and a roughly one-stop advantage at 72mm (35mm equivalent) with the LX15 over the LX7.

In other words, in terms of low light performance, the LX15 compares to the LX7 about like a FF camera would compare to an APS-C camera. That's a significant difference in low light.

That's also before one considers the possible effect of the four year difference in technology between the LX7 released in 2012 and the LX15 released in 2016. That may or may not be a large factor, as improvement in sensor technology has begun to plateau a bit. In general, comparable models between, say, 2008 and 2012 would be expected to show more improvement than comparable models between 2012 and 2016. But every case is different.

As to the price disparity, the cheapest FF cameras are at least twice as much as the cheapest APS-C cameras. The Canon EOS 6D Mark II lists for $1,800 but currently sells for about $1,300 with a $500 instant rebate in the U.S. The Canon EOS 77D lists for $750 and sells for about $700. (The 77D is fairly comparable in features and controls to the 6D Mark II. They both were introduced in 2017, they both have similar 45-point AF systems, etc. One can get Rebel/xx0D models cheaper, but they don't have the same level of controls and other features.)

Is future proofing a big factor here?

There's no such thing as future proofing, especially when it comes to cameras. By the time one model hits the market, the replacement for that model is already being anticipated by many gearheads obsessed with cameras instead of with photography.

¹ Equivalent aperture (in 135 film terms) is calculated by multiplying lens aperture with crop factor (a.k.a. focal length multiplier).