As someone who has been in and out of Gaydon in various capacities since 2010, let me give you my insider's view of what went wrong with the Jaguar brand.
in 2010 when I first joined, X250 (XF) had not long been launched, and X351 (final XJ) was launching. This was in the immediate aftermath of the sale from Ford to Tata. At the time there was a huge push to improve brand awareness and more importantly, quality. This was largely successful, since Jaguar was near the top of the JD Power survey for the next couple of years.
Development of F-Type was ongoing at this point, and due to the budget issues with the company nearly folding, development suffered and massive compromises had to be made in order to deliver a smaller 2-seat sibling to XK. F-Type was essentially a cut and shut XK with compromised dynamics as a result. Plus the cheap V6 concept that was a V8, but a V6, but actually a V8. Quality was eroded during this time, but, brand awareness and marketing meant OK-ish sales of F-Type.
Cue Dr. Ralf Speth joining the company and his grand designs to grow JLR into a competitor to the German OEMs. A grand idea, with lots of potential, but poor implementation due to poor support withing the Boardroom for Jaguar. At this point you have to remember that Jaguar and Land Rover had been thrust together under Fords PAG, so there was still a lot of rivalry between relics from Rover and people from "The Jag". Unfortunately, the Board consisted of many of the aforementioned relics, and even Dr. Speth was not supposedly overenthused by Jaguar. Hence the brand was suffering from lack of love from the top brass.
Now we come to the continued growth and revival of Jaguar. D7a platform was envisioned as a modular architecture for medium-sized vehicles (C,D & E segment really). Hence X760 (XE), X260 (new XF), X761 (F-Pace) plus some Land Rovers and Tata products. There were also a couple of plans for X764 (XE coupe/convertible) and a couple others I can't remember. At this point all the focus was on getting the products developed, at a cost, en mass, on time. The previous focus on quality went out the door. This combined with another cringey launch (XE - horrible), poor interior material quality choices, a confusing model naming convention (R-Dynamic, Portfolio etc), lack of Driver focussed options for a "Sports Saloon" (manual transmission, LSD, etc) meant the products were sub-par compared to Ze Germans. Finally, and most crucially, the finance deals were extremely uncompetitive compared to other OEMs. A friend who worked at Ascot JLR selling Jags told me he was 25% more expensive per month for a Lease XE compared to the equivalent BMW 3 series.
So XE was poor quality, cramped inside, lacking in exciting features, and critically had no halo model, no M3 or RS4. And people were asking in the dealerships for them, JLR just didn't listen ("there's no business case" being a consistent excuse at Gaydon)
X260 (XF replacement) is a great car to drive, spacious and comfortable. But the launch was invisible. Most people didn't even know there was a new XF. And then again, the same issues (mostly) as XE; odd models, no Halo variant, fairly bland colour palatte.
X761 was marketed brilliantly, and sold well (for Jag), but suffered from the same quality oversight.
X590 (I-Pace) had all the backing you could want within the business in order to be first to market with an EV (against Audi, Porsche etc). Which it achieved. But then they did nothing. No follow up models, no additional body variants (top-hatting is relatively cheap once a platform is understood), no EV variants of other D7a models (unlike Audi who mastered this idea).
On all of the above, build quality, component quality, reliability of Infotainment (shockingly bad), NVH etc were all lacking due to lack of rigour by the Board in ensuring Engineering robustness. Instead it was "get it done, get it out the door, we have gateways to hit". And to this day, gateways at JLR are just a whistling noise you hear as you sail through them with issues Trackers turned green for one day, with the hope of resolution at a later date.
So in a nutshell, if the above had been bought into properly by the board, had rigorous leadership and focus on quality, an understanding of the target customer, the right features on a given vehicle model, model naming that was understood (was the hell is Portfolio anyway?!), a financial package that was competitive, and product launches that were exciting, consistent and advertised properly, then I think the products would have spoken for themselves. And last but not least, Dealer Service that was on top of its game. The Tech's I have had to deal with at the Dealers sometimes shock me. There are some who are genuinely brilliant, but some of them clearly struggle to spell their own names.