An old piano is not only inferior to a new instrument due to its wear and tear over the years, but lacks so much innovative design and precision manufacturing.
Wear and tear aside, to focus on the difference in design, older pianos were typically built with animal and water-based glues, which do not cope so well with fluctuations in humidity and temperature. Structural, tonal and mechanical wooden parts have more moisture content, as the older pianos were built for a cold, damp room, with a fireplace. Central heating creates a consistently comfortable temperature, but, unfortunately for early pianos, an artificially dry environment too.
The wooden action parts, soundboard and bridge, wrest plank (tuning pin block), keys and key-frame, and backposts/beams in quality innovative modern pianos, such as Yamaha pianos, are ‘seasoned’ (dried) prior to assembly in a computer controlled kiln so the wood is suitable for a modern home. Felts and leathers covering action and keyboard components can be specially treated, the use of aircraft grade glues and multi-laminated hard-woods, all offer substantially more resilience to humidity and temperature fluctuations.
Tuning stability, consistency and reliability in touch response and retention of the soundboard’s crown for continued tonal quality, are all vital factors in ensuring trouble-free performance from your piano.
The piano has evolved over hundreds of years. Major improvements in design for a more efficient and responsive touch, extended dynamic range, broader and more contrasting tonal sonority, consistent pronunciation, durability and reliability are still being implemented today. Therefore, a 1920’s grand by a prestigious manufacturer may indeed perform to a much lower standard, as well as suffering the structural strain of up to 20metric tonnes for such a duration.
As detailed above, older pianos are inferior in design and not built for a modern home. Also, an older piano that has been extensively restored to the highest standards will still sound ‘old’ and, worst of all, the structural areas such as the frame and the beams / backposts have been subject to huge amounts of string tension, and the soundboard and bridge which deliver the resonance are likely to split as they suffer great amounts of down-bearing from the strings. The frame could develop hairline cracks when re-strung, which would involve costly repair. An older piano simply cannot last as long as a quality new one.