Both. Semi vs concrete, depreciating vs durable assets. Durable linear assets you upgrade switches to improve fiber/rail which is where most of the investment was in. GPUs you replace the racks which is where most of the investment is in. Either way it cannot be stretched the same way materially and most important economically, i.e. new chips with better power efficiency = running old chips is literally losing money squatting on an data center slot. There is very little reason to believe new chips will cost more than legacy (current chips) for the simple reason much of currnet chips are acquired on scarcity pricing, i.e. Nvidia margins went from 50% to 70%. That 20% is massive capex premium that is not going to be competitive if bubble pops and Nvidia has to sell new hardware on commodity pricing, new hardware that is in all likelihood also going to be more compute efficient in terms of power (opex). Even if you stretch existing compute past 3-5 to 10 years it is still closer to tuplids than rail or fiber in terms of economically productive timescale.

TLDR old durable infra tends to retains positive residual value because they're not easy to replace economically/frequently, old compute has negative residual value because they are easy to replace economically/frequently.