I would agree that US defense spending still has room to increase, particularly when measured in terms of percentage of GDP, compared with those in the Cold War era.
But I also want to caution against going too far using this yardstick, because it can be misleading and confusing. Defense industry is mostly manufacturing and industrial scale and capabilities. In that sense, the US manufacturing and industrial capacity and capability have declined significantly in relative terms compared with those in the '50s, '60s, '70s and even '80s. Looking from a different angle, during the Cold War, the US's chief geopolitical rival, the USSR's industrial output and capacity were far below those of the US (<50%). Today, as the US's primary geopolitical competitor, China's industrial value-add is equal to the combined industrial value-add of the next three largest industrial nations: the US, Japan and Germany.
Put it simply, all GDP are not created equal, at least when it comes to defense spending. GDP of today's US is more service-driven, much de-industrialized and substantially inflated by dollar's reserve status. It's not immediately clear that the US has a clear edge should it plunge into an arm race against China.
I agree that GDP, particularly nominal GDP, is an imperfect measure, however in decades of reading I have yet to see a credible alternative measure proposed, even by those with clear incentives to do so.
This discussion has begun to wander significantly off topic. The question is if US Congress' proposal to order (and, by extension, produce and deliver) three Burkes per year for five years is achievable. I contend that it is achievable because it is more or less what was occurring before Zumwalt and LCS entered the picture. Indeed, that is the problem that this proposal from Congress anticipates, i.e. the need to replace ships (including early Burkes) that were constructed at a high cadence decades ago, and will therefore reach retirement age at a similarly cadence, requiring replacement at a similar cadence.
Michaelsinodef then raised the question, looking beyond industrial capacity at HII, BIW and the broader Burke supply chain, if cost and budgets might be a limiting factor. My answer to this was no, because I anticipate that US military spending will continue to increase going forward. That is not to say I anticipate a drastic increase (though I would certainly not rule it out, one never knows when Reagan 2.0 is going to appear on the American political scene), but at the end of the day we are talking about an additional USD $2.2bn (i.e. one additional Burke per year) in an annual Navy budget of some USD $230bn, including $53bn for procurement (FY2023 DoD request) Moreover, these mooted additional Burkes are explicitly conceived as an alternative to additional FFG-62 frigates, hence the actual annual increase in expenditure from a 3 Burke/1 Constellation structure compared to a 2 Burke/2 Constellation structure would be around $1.1-1.2bn/yr (unit costs from USN's April 2022 Long-Range Shipbuilding Plan). Ultimately it is Congress that determines both the overall budget and which ships are and are not funded, and the Navy is left to work within those parameters. Often this means that Congress continues to order stuff that USN has sought to wind down, such as more LCS and Super Hornets. If Congress wants three Burkes per year, they will get three Burkes per year, even if that means USN has to make cuts in areas not under Congressional control, e.g. maintenance, training, deployment schedules.